Allow ctlplane network or ID, and default to "ctlplane"

Allow ctlplane network or ID, and default to "ctlplane"

Currently you always have to pass the ctlplane ID because we're still
using the deprecated network_id property for the neutron port resource.

Since Juno, heat has supported a "network" property, which is used
elsewhere, e.g the nested port stacks, so switch to using it in the
overcloud-without-mergepy template, and flip the default to a more
useful "ctlplane" vs an empty string.

This means the stack create should just work on commonly documented
deployments without requiring any parameter.

Change-Id: Ifcea36d26b795c5e8b80accd8112e23b254127be