We're going to add new alembic-based migrations shortly. These will live
in the 'keystone.common.sql.migrations' module. Prepare for this by
moving the existing migrations from ''keystone.common.sql' into a common
'keystone.common.sql.legacy_migrations' module.
Change-Id: I5ab7b010b21268977f73738e895bbd21442e9455
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>