Remove new constraint from migration downgrade.

Remove the foreign key constraint for user_id. Whilst it is a foreign
key the constraint was not set in the previous schema. Also sets the
columns names to unicode as they were in previous schema.

Fixes: bug 1169381
Change-Id: I9b1ac5305622d68a7e124ba8100a4b8166b43af0

keystone/common/sql/migrate_repo/versions/020_migrate_metadata_table_roles.py

@@ -70,12 +70,11 @@ def downgrade(migrate_engine):
         'metadata',
         meta,
         sql.Column(
-            'user_id',
+            u'user_id',
             sql.String(64),
-            sql.ForeignKey('user.id'),
             primary_key=True),
         sql.Column(
-            'tenant_id',
+            u'tenant_id',
             sql.String(64),
             primary_key=True),
         sql.Column('data',