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