ml2 provides no information if there is no suitable mech_driver found during port binding

If there is no suitable mech driver found, ML2 just make port bind_failed and write uninformative message in the log:

2017-01-12 13:56:46.691 3889 ERROR neutron.plugins.ml2.managers [req-d9d956d7-c9e9-4c1b-aa1b-59fb974dd980 5a08515f35d749068a6327e387ca04e2 7d450ecf00d64399aeb93bc122cb6dae - - -] Failed to bind port f4e190cb-6678-43f6-9140-f662e9429e75 on host d02c7361-5e3a-4fdf-89b5-f29b3901f0fc for vnic_type baremetal using segments [{'segmentation_id': 21L, 'physical_network': u'provision', 'id': u'6ed946b1-d7f6-4c8e-8459-10b6d65ce536', 'network_type': u'vlan'}]

I think it should report reason for this to admins more clearly, saying that no mechanism driver found to bind port.

In my case it was: INFO neutron.plugins.ml2.managers [-] Loaded mechanism driver names: [], which was hard to debug due to lack of any information from neutron-server (even in debug mode!).

version: 2:9.0.0-0ubuntu1~cloud0