Change of DHCP enable flag is not triggering an update.
Change of DHCP enable flag is not triggering an update.

Handle change of dhcp enable flag to trigger an update in the interface.
closes-bug: #1571998

Change-Id: I3120ea3711c8fba897f33c4b3419aa7c4fc0ded6
(cherry picked from commit 5f01ff31d8771e080e1345f22207957482b817ea)

codilime/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,codilime/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,codilime/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,codilime/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,codilime/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,codilime/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller