Fix the bug of missing constraint

1. What is the problem
In the resource routing table, there does not exist unique constraint
on top_id and pod_id.

2. What is the solution to the problem
Add constraint on the two columns.

3. What the features need to be implemented to the Tricircle
No new feature is implemented.

Change-Id: I13ce66a270de280ba13b0733cc1f39648e72d0eb

tricircle/db/migrate_repo/versions/002_resource.py

@@ -249,6 +249,10 @@ def upgrade(migrate_engine):
         sql.Column('resource_type', sql.String(length=64), nullable=False),
         sql.Column('created_at', sql.DateTime),
         sql.Column('updated_at', sql.DateTime),
+        migrate.UniqueConstraint(
+            'top_id', 'pod_id',
+            name='cascaded_pods_resource_routing0top_id0pod_id'
+        ),
         mysql_engine='InnoDB',
         mysql_charset='utf8')