undercloud_post: fix subnet name
undercloud_post: fix subnet name

The default control plane subnet name is "ctlplane-subnet", so let's
create the right subnet for the containerized undercloud.

Note: the subnet can't be overriden (yet) but for now we rely on the
default.

Change-Id: I15954bced81ef6c3e1a1f4a73bc989f33d08d6f7

openstack/tripleo-heat-templates,dprince/tripleo-heat-templates,openstack/tripleo-heat-templates,dprince/tripleo-heat-templates