Fix the migration adding a UC to agents table

The migration script mistakenly assumes that all core
plugins use agents extension, which is not true (e.g.
plumgrid and bigswitch don't).

Apply this migration script only for plugins that are
stated in the original migration script adding agents
table (511471cc46b_agent_ext_model_supp.py).

Related-Bug: #1254246

Change-Id: I7915ef8d183782eb5d46ac47f45014aa9e9640fb

Conflicts:
	neutron/db/migration/alembic_migrations/versions/1fcfc149aca4_agents_unique_by_type_and_host.py