Do set DNS on overcloud subnets

It turns out we do need to set DNS on the overcloud subnet. It should be
the .2 of the CIDR under test and not the _admin_net's .2. There seems
there are security group rules in the way.

zaza/openstack/configure/network.py

@@ -172,6 +172,10 @@ def setup_sdn(network_config, keystone_session=None):
         network_config.get("private_net_cidr"),
         subnetpool=subnetpool,
         ip_version=ip_version)
+    openstack_utils.update_subnet_dns(
+        neutron_client,
+        project_subnet,
+        network_config["external_dns"])
     openstack_utils.plug_subnet_into_router(
         neutron_client,
         network_config["router_name"],