Handle racing condition in OFC port deletion

Handle racing condition in OFC port deletion

Multiple delete_port operations can run in parallel.
For such case later operation(s) will receive 404 (NotFound)
error from OFC or NotResultFound sqlalchemy exception.
These are valid exceptions and they should be ignored
in delete_port operations.

Closes-Bug: #1281574

OFCConsistencyBroken is renamed to OFCMappingNotFound
because when multiple delete_port operations run in parallel
OFCConsistencyBroken can occur and it is a valid case
so the excepion name looks inappropriate.

Change-Id: I1511d55994c88b8828f0ff62610c18ddc6dfac8f