Add dhcrelay monitor via supervisor to restart

Dockerctl can start dhcrelay, but not ensure it
stays running. Supervisord can handle this task,
so it should watch the pid of dhcrelay and trigger
restart if it exits.

Change-Id: I75909d25418e625a2b154125e9a76e22c7443e47
Closes-Bug: #1324152
(cherry picked from commit acf69bd4c1441458843a59858038d11ad2bbe9cc)
Merge "Add dhcrelay monitor via supervisor to restart" into stable/5.0