[Services] Restart SNMP service upon unexpected critical process exit.

[yozhao101]

• What I did
  Restart SNMP service if one of critical processes running in SNMP container exited or crashed abnormally.

• How I did it
  Generally I follow the framework created by Joe to implement this feature in SNMP container.
  First, add supervisor-proc-exit-listener event listener option in Supervisord configuration file in SNMP docker container. Supervisord will read a list of critical processes for which to monitor the unexpected crashed and exited.
  Second, configure snmp.service to always auto-restart the service if it stops, with a delay of 30 seconds. Also set a rate limit of 3 restarts within 20 minutes (1200 seconds).

• How to verify it
  On your switch device, please use docker ps command to list all running docker containers.
  Then use docker exec -it container_id /bin/bash to login target container. Typing top command
  on the shell will display all the processes dynamically and you will spot the process id of one
  of the critical processes. Finally type the command kill -9 process_id to terminate one process.
  After exiting the container, you can use watch -n 1 docker ps to dynamically see the restart
  of SNMP container.

[jleveque]

Retest vs please