Don't swallow exceptions in deallocate_port_for_instance

This uses the new _delete_ports method to (1) still
ignore 404 errors but also (2) raise up non-404 errors
from the neutron client when deleting the port fails as
we shouldn't be masking over this since the nova-api
call wouldn't be failing even though the underlying
operation failed.

Change-Id: Id05ea47ba03e0b60ae3a296920159e8189f9dd55
Related-Bug: #1292243
Merge "Don't swallow exceptions in deallocate_port_for_instance"