Load tables of service plugins when migration auto generation

When we run "neutron-db-manage revision --autogenerate", it detects
table deletion of service plugins even when they are configured.
We need to load tables of service plugins to teach alembic such
tables exist.

Change-Id: I9186d36496069c8253683bd200326b80a6565fa6
Closes-Bug: #1218338
Merge "Load tables of service plugins when migration auto generation"