Update test_vifs to ignore ovs errors on del-port

nova has changed upstream. Now, when calling ovs-vsctl to delete a port
they explicitly short circuit the deletion if the port does not exist.
This broke nova-docker's mock call. Fix it so tests pass.

Reference:
https://github.com/openstack/nova/commit/476dc5efcdb11c9859b062dc69a8c205e69be861

Closes-Bug: #1324175
Change-Id: I920536b27a7e7fd30e7d80148d38f7ef6877c707