Enable caching of host and group ansible variables

[wangxin]

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?

Loading variables from inventory files is a kind of expensive operation. Caching them could further improve testing performance.

How did you do it?

This change enabled caching of below items:

• Ansible InventoryManager instance
• Ansible VariableManager instance
• Host visible variables
• Group visible variables
• Test server variables
  Another change is to use a lock to guard writing to cache.

How did you verify/test it?

Tested the change using a test script like below:

import logging

from tests.common.utilities import get_inventory_files
from tests.common.utilities import get_host_vars
from tests.common.utilities import get_host_visible_vars
from tests.common.utilities import get_group_visible_vars
from tests.common.utilities import get_test_server_vars

logger = logging.getLogger(__name__)


def test_case1(duthost, request):
    inv_files = get_inventory_files(request)
    vars = get_host_vars(inv_files, duthost.hostname)
    logger.info(str(vars))


def test_case2(duthost, request):
    inv_files = get_inventory_files(request)
    vars = get_host_visible_vars(inv_files, duthost.hostname)
    logger.info(str(vars))


def test_case3(duthost, request):
    inv_files = get_inventory_files(request)
    vars = get_group_visible_vars(inv_files, 'sonic')
    logger.info(str(vars))


def test_case4(duthost, request):
    inv_files = get_inventory_files(request)
    vars = get_test_server_vars(inv_files, 'server_1')
    logger.info(str(vars))


def test_case5(duthost, request, enum_frontend_dut_hostname, enum_asic_index):
    inv_files = get_inventory_files(request)

Any platform specific information?

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

Documentation

[sanmalho-git]

@wangxin - can you expedite merging this PR as would like to use that in my optimization for pytest_generate_test PR as well.

[wangxin]

Force pushed to resolve conflicts.

[bingwang-ms]

retest this, please