VMware NSX: Fix db integrity error on dhcp port operations

If the dhcp port and network disappear, ensure that
the integrity constraint violation that results from
inserting the neutron/nsx port mapping to the DB does
not propagate the exception all the way through, but
instead is caught and handled correctly.

Closes-bug: #1265472

Change-Id: I3ae072729d579def0bd9658241eda1011a3dcfa0
Merge "VMware NSX: Fix db integrity error on dhcp port operations"