Fix ml2 db migration of subnetroutes table

The ml2 db migration path doesn't create subnetroutes table. Therefore
installations were partially relying on automatic schema generation.

This patch fixes the migration path.

Change-Id: I3060dd9538b629ad7277af28008ddde27926dcc4
Closes-Bug: #1290255
Merge "Fix ml2 db migration of subnetroutes table"