[multi-asic]: Add function to load database config

[SuvarnaMeenakshi]

Description of PR

Summary:
Fixes # (issue)

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• Test case(new/improvement)

Back port request

• 201911

Approach

What is the motivation for this PR?

With the changes in sonic-net/sonic-buildimage#8173, the applications that connect to namespace dbs have to load global database configuration. Due to this change, the sonic-py-common.multi_asic functions that connect to database will fail if database configuration is not loaded in ansible library functions.

How did you do it?

Add a new module_utility library function to load the right db configuration based on single or multi-asic platform.
Invoke this function in port_util and lag_facts where mutli_asic library function is invoked which connects to db.

How did you verify/test it?

Bring up a multi-asic VS testbed.
./testbed-cli.sh -t vtestbed.csv -m veos_vtb -k ceos add-topo vms-kvm-four-asic-t1-lag password.txt
Before adding this change, failure seen while running test_bgp_fact.py

./run_tests.sh -c "$tests" -d vlab-08 -f vtestbed.csv -i veos_vtb -n vms-kvm-four-asic-t1-lag -e --disable_loganalyzer -u
        if (res.is_failed or 'exception' in res) and not module_ignore_errors:
>           raise RunAnsibleModuleFail("run module {} failed".format(self.module_name), res)
E           RunAnsibleModuleFail: run module minigraph_facts failed, Ansible Results =>
E           {
E               "changed": false, 
E               "failed": true, 
E               "invocation": {
E                   "module_args": {
E                       "filename": null, 
E                       "host": "vlab-08", 
E                       "namespace": null
E                   }
E               }, 
E               "msg": ":- validateNamespace: Initialize global DB config using API SonicDBConfig::initializeGlobalConfig\nTraceback (most recent call last):\n  File \"/tmp/ansible_minigraph_facts_payload_XTvxQr/__main__.py\", line 823, in main\n    results = parse_xml(filename, m_args['host'], namespace)\n  File \"/tmp/ansible_minigraph_facts_payload_XTvxQr/__main__.py\", line 636, in parse_xml\n    port_alias_to_name_map, port_alias_asic_map = get_port_alias_to_name_map(hwsku, asic_id)\n  File \"/tmp/ansible_minigraph_facts_payload_XTvxQr/ansible_minigraph_facts_payload.zip/ansible/module_utils/port_utils.py\", line 18, in get_port_alias_to_name_map\n    namespace_list = multi_asic.get_all_namespaces()\n  File \"/usr/local/lib/python2.7/dist-packages/sonic_py_common/multi_asic.py\", line 226, in get_all_namespaces\n    config_db = connect_config_db_for_ns(namespace)\n  File \"/usr/local/lib/python2.7/dist-packages/sonic_py_common/multi_asic.py\", line 40, in connect_config_db_for_ns\n    config_db = swsscommon.ConfigDBConnector(namespace=namespace)\n  File \"/usr/lib/python2.7/dist-packages/swsscommon/swsscommon.py\", line 2076, in __init__\n    super(ConfigDBConnector, self).__init__(use_unix_socket_path = use_unix_socket_path, namespace = namespace)\n  File \"/usr/lib/python2.7/dist-packages/swsscommon/swsscommon.py\", line 1698, in __init__\n    for db_name in self.get_db_list():\n  File \"/usr/lib/python2.7/dist-packages/swsscommon/swsscommon.py\", line 1634, in get_db_list\n    return _swsscommon.SonicV2Connector_Native_get_db_list(self)\nRuntimeError: :- validateNamespace: Initialize global DB config using API SonicDBConfig::initializeGlobalConfig\n"
E           }

complex_args = {'host': 'vlab-08', 'namespace': None}
filename   = '/data/sonic-mgmt/tests/common/devices/sonic.py'
function_name = 'get_extended_minigraph_facts'
index      = 0
line_number = 1210
lines      = ["        mg_facts = self.minigraph_facts(host = self.hostname, namespace = namespace)['ansible_facts']\n"]
module_args = ()
module_async = False
module_ignore_errors = False
previous_frame = <frame object at 0x555b4ff69d30>
res        = {u'failed': True, u'exception': u'WARNING: The below traceback may *not* be re...Initialize global DB config using API SonicDBConfig::initializeGlobalConfig\n'}
self       = <SonicHost> vlab-08
verbose    = True

After fix:
=== Running tests individually ===

/usr/local/lib/python2.7/dist-packages/ansible/parsing/vault/__init__.py:44: CryptographyDeprecationWarning: Python 2 is no longer supported by the Python core team. Support for it is now deprecated in cryptography, and will be removed in a future release.
  from cryptography.exceptions import InvalidSignature
========================================= test session starts =========================================
platform linux2 -- Python 2.7.17, pytest-4.6.5, py-1.9.0, pluggy-0.13.1
ansible: 2.8.12
rootdir: /data/sonic-mgmt/tests, inifile: pytest.ini
plugins: metadata-1.10.0, forked-1.3.0, html-1.22.1, repeat-0.9.1, xdist-1.28.0, ansible-2.2.2
collected 4 items                                                                                     

bgp/test_bgp_fact.py::test_bgp_facts[vlab-08-0] PASSED                                          [ 25%]
bgp/test_bgp_fact.py::test_bgp_facts[vlab-08-1] PASSED                                          [ 50%]
bgp/test_bgp_fact.py::test_bgp_facts[vlab-08-2] PASSED                                          [ 75%]
bgp/test_bgp_fact.py::test_bgp_facts[vlab-08-3] PASSED                                          [100%]

Tested the same with 201911 VS image to ensure that changes do not impact test cases running on 201911 image.

Any platform specific information?

Supported testbed topology if it's a new test case?

Documentation

[arlakshm]

can we do load_db_config just once in the beginning, iso doing for every portchannel or every NS?

[SuvarnaMeenakshi]

Fixed as suggested.

[arlakshm]

as comments

[arlakshm]

/Azp run

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