NSX: Fix router-interface-delete returns 404 when router not in nsx

Previously, if one would run router-interface-delete and the router
was not found in NSX. Neutron would raise a 404 error and remove the interface
from the database. In this case it would be better if we did not raise
a 404 error as the caller shouldn't be aware that the backend is out of sync.

Closes-bug: #1291690

Change-Id: I2400f8b31817e6dd4bc8aa08f07e1613fc2deeae
Merge "NSX: Fix router-interface-delete returns 404 when router not in nsx"