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

[yozhao101]

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

• How I did it
  Generally I follow the framework created by Joe to implement this feature in NAT container.
  First, add supervisor-proc-exit-listener event listener option in Supervisord configuration file in NAT docker container. Supervisord will read a list of critical processes for which to monitor the unexpected crashed and exited.
  Second, configure nat.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 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 database container.

[jleveque]

Retest vsimage please

[rlhui]

@yozhao101, @kirankella, @AkhileshSamineni, wondering what'd be the impact to existing NAT entries/connections when the NAT service is restarted? Thanks.

[AkhileshSamineni]

@rlhui , Whenever NAT service/docker is restarted, the iptables and conntrack entries are cleared and also sends a cleanup notification to OA to delete all the NAT entries from appl-db and asic-db.
This is handled by the sigterm handler in natmgr code.