Add nova.db.migration.db_initial_version()

Updates Nova to use a new function called db_initial_version()
instead of the INIT_VERSION constant. This allows us to
move the top level INIT_VERSION = 132 into the
sqlalchemy migration.py implementation where it belongs.

Also, includes changes to do the same for the Nova
baremetal migrations.

Change-Id: I10dacdebb50d3121e3bfd094e40138807233aa64
Merge "Add nova.db.migration.db_initial_version()"