[system-health] [202012] No longer check critical process/service status via monit

[Junchao-Mellanox]

Backport #9068 to 202012

Why I did it

Command monit summary -B can no longer display the status for each critical process, system-health should not depend on it and need find a way to monitor the status of critical processes. The PR is to address that. monit is still used by system-health to do file system check as well as customize check.

How I did it

1. Get container names from FEATURE table
2. For each container, collect critical process names from file critical_processes
3. Use “docker exec -it <container_name> bash -c ‘supervisorctl status’” to get processes status inside container, parse the output and check if any critical processes exit

How to verify it

1. Add unit test case to cover it
2. Adjust sonic-mgmt cases to cover it
3. Manual test

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106

Description for the changelog

A picture of a cute animal (not mandatory but encouraged)