Use full plugin class paths for Neutron

Neutron database migration utility doesn't support neutron.conf
where core or service plugins are configured via their aliases,
so full plugin class paths are required

This change is required for https://review.openstack.org/#/c/97398
Closes-bug: #1335149

Change-Id: I5ab348be6c9590a9317211178ca4cff4bb152be2