Remove dependency on config file for db check_migration
Remove dependency on config file for db check_migration

The alembic check_migration command just needs a database connection
to do the check, so we can avoid referring to a config file which may
be in different places depending on how neutron is installed.

Change-Id: I2d8043538ce8cf9568a84d256dbe8e6d3c5e5e3d

openstack/neutron-vpnaas,openstack/neutron-vpnaas