Fix self.fields on API Port object

All fields from objects.Port were being added to self.fields in the
API Port object. Because of this, when someone would POST, there'd be an
entry for {'id': None} in the dictionary passed to dbapi.create_port(). We
should only set fields we're exposing.

This also required fixing PATCH to not try to look at fields not set on
the API Port object when mapping to objects.Port.

Change-Id: I7f163d42d1298ce4ba62b1b7d637fb0a4e3409ce
Merge "Fix self.fields on API Port object"