nicira: fix db integrity error during port deletion

nicira: fix db integrity error during port deletion

Due to the fact that plugin port operations are not
transactional (as they involve non synchronized DB
and Controller operations), concurrent (interleaved)
port requests may accidentally cause the insertion
of the mapping entry (neutron-port-id, nvp-port-id)
more than once. In case this occurs, it's safe to
expect the failure and continue the normal process
of the operation being requested.

Closes-bug: #1265081

Change-Id: Ifcf5b453fa08145df844c2de3cbb08bf2f4baa59