[platform] Implement platform phase 2 cases

[wangxin]

Description of PR

Summary:
Fixes # (issue)

Implement the SONiC platform phase 2 test cases using the pytest-ansible framework. This implementation is based on the phase 1 scripts: #915

The SONiC platform test cases are documented here:
https://github.com/Azure/SONiC/blob/master/doc/pmon/sonic_platform_test_plan.md

Test cases implemented in phase 2:

Test Case	Script	Common for all vendors?
Case 1.5 Sequential syncd/swss restart	tests/platform/test_sequential_restart.py	Yes
Case 1.6 Reload configuration	tests/platform/test_reload_config.py	Yes
Case 1.7 COLD/WARM/FAST reboot	tests/platform/test_reboot.py	Yes

Some extra helper scritps are also included in this PR. The helper scripts contain re-usable code for checking various status:

Reusable script	Purpose
check_critical_services.py	Check status of critical services for eg: wss, syncd, bgp, etc. Used by the test cases testing restart/reload/reboot.
check_hw_mgmt_service.py	Check status of hw-management, thermal control process, fan speed, PWM setting. Used by the test cases testing restart/reload/reboot.
check_interface_status.py	Check status of interfaces. Used by the test cases testing restart/reload/reboot.
check_sysfs.py	Check the symbolic links under /var/run/hw-management. Used by the test case testing SYSFS.
check_transceiver_status.py	Check the status of xcvrd information in DB. Used by the test case testing xcvrd and restart/reload/reboot.

Type of change

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

Approach

How did you do it?

Implement the SONiC platform phase 2 test cases using the pytest-ansible framework and follow the design of PR #802

How did you verify/test it?

Tested on the mellanox platform

Any platform specific information?

The scritps contain some platform specific checking. This pattern is used for such checking:

if sonic_asic_type in ["mellanox"]:
   do_something()
else:
   do_something_else()

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

All current test topologies are supported:
[t0, t0-16, t0-56, t0-64, t0-64-32, t0-116, t1, t1-lag, t1-64-lag, ptf32, ptf64]

Documentation

The SONiC platform test plan:
https://github.com/Azure/SONiC/blob/master/doc/pmon/sonic_platform_test_plan.md