[pytest] Improve 'cached' decorator

[lolyu]

Description of PR

Summary:
Fixes # (issue)

Type of change

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

Approach

What is the motivation for this PR?

1. The original cached decorator has limitations that it only supports being used for methods of class AnsibleBaseHost and its derivatives.
2. FactsCache returns None in either the FactsCache fails to find cached facts or the decorated function returns None.
3. utils functions in tests/common/utilities that try to retrieve host variables share a similar call pattern that the arguments all have hostname defined and have the same logic to check the inventory in cache reading or add the inventory in cache writing.

How did you do it?

1. Add a zone_getter argument to the cached decorator to let it receives a customizable function to get the zone.
2. When FactsCache fails to find cached facts, let it returns FactsCache.NOTEXIST instead of None(None is a valid return from the decorated function)
3. Add arguments after_write and before_read to the cached decorator to let it have extra functionalities to process the facts after the cache reading/before the cache writing.

How did you verify/test it?

(Pdb) utils.get_group_visible_vars(inv_files, "server_17").keys()
[u'ansible_become_password', u'eos_login', u'skip_ceos_image_downloading', u'ceos_image', 'ansible_check_mode', u'vm_mgmt_gw', u'sonic_default_passwords', u'telemetry_certs', 'ansible_diff_mode', 'ansible_forks', u'eos_root_user', 'playbook_dir', u'eos_default_password', u'vm_console_base', 'ansible_playbook_python', u'mux_simulator_port', u'external_port', 'ansible_facts', u'eos_root_password', 'ansible_verbosity', 'inventory_hostname', u'k8s_master_login', u'secret_group_vars', 'omit', u'host_var_file', u'topologies', u'str2', u'ceos_image_filename', u'memory', u'mgmt_bridge', u'proxy_env', u'eos_default_login', u'ceos_image_orig', u'max_fp_num', u'ptf_bp_ipv6', 'inventory_file', u'switch_login', u'secret_host_vars', u'mgmt_prefixlen', u'skip_image_downloading', u'secret_vars', 'group_names', u'corefile_uploader', u'lab', u'mgmt_gw', u'ansible_user', 'groups', u'hwsku_map', u'restapi_certs', u'ansible_host', u'sonic_image_filename', u'sonic_login', 'ansible_inventory_sources', u'vm_images_url', u'cd_image_filename', u'eos_password', u'root_path', u'example_ixia', 'inventory_dir', u'ptf_bp_ip', u'sonic_password', u'hdd_image_filename', 'ansible_version', u'str', 'inventory_hostname_short', u'supported_vm_types', u'k8s_master_password', u'ansible_password']
(Pdb) utils.get_group_visible_vars(inv_files, "server_17")["mux_simulator_port"]
8080
(Pdb) utils.get_host_vars(inv_files, "str2-7050cx3-acs-09")
{u'pdu_host': u'pdu-119', 'inventory_file': u'/data/repo/cache/refact/sonic-mgmt/ansible/str2', u'ansible_host': u'10.3.147.20', u'hwsku': u'Arista-7050CX3-32S-D48C8', 'inventory_dir': u'/data/repo/cache/refact/sonic-mgmt/ansible'}

Any platform specific information?

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

Documentation

[lgtm-com]

This pull request introduces 2 alerts when merging a7aeaf09311c7d51efbc1d8213931c16981ae541 into b4b6d21 - view on LGTM.com

new alerts:

• 2 for Unused import

[lolyu]

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[bingwang-ms]

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[lolyu]

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[lolyu]

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[lolyu]

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