Handle failures on update_dhcp_port

Ensure exceptions due to conflicting
state of network or subnet resources
are dealt with by the dhcp agent.

Closes-bug:  #1253344
Related-bug: #1243726

Change-Id: I4fd51442c034fabc91d5a3f065f4df98f5fad35b
Merge "Handle failures on update_dhcp_port"