Run Neutron migrations after creating schema

Partial-Bug 1207402

This patch executes Neutron DB migrations (based on alembic)
before starting the Neutron services, similar to what happens
for most of other openstack projects managed by devstack.
This will ensure devstack always creates correctly version-stamped
Neutron databases.

Change-Id: Ia941c426a1563bcc4cb5eae64ea30c0bf7677220
Merge "Run Neutron migrations after creating schema"