Catch PortNotFound exception during get_dhcp_port

Close-bug: 1252437

Change-Id: I3a7954b869b4f5b909c806f8e47180e1ba6ba374
(cherry picked from commit cbe91fda7b416717a23cf7b29adf8201a8c14822)
Merge "Catch PortNotFound exception during get_dhcp_port" into stable/havana