Update dhcrelay config via dhcrelay_monitor

dhcrelay_monitor restarts dhcrelay if it is not started
or dies. It should also update its config for the current
IP of Cobbler because it could change after reboot.

Change-Id: Id6b74169b637fab8c5df03477774ad88ce733a3d
Closes-Bug: #1333362
Merge "Update dhcrelay config via dhcrelay_monitor" into stable/5.0