Support adding extra routes when creating subnets

os_cloud_config.neutron.create_subnet will already add a route for
the special cloud-init IP to the metadata server, but we should
allow operators to specify extra routes when creating subnets.

Change-Id: Ic755a31b639454b3b67502090b57ac31eaee45f0
Merge "Support adding extra routes when creating subnets"