In the case where there are multiple private networks, don't actively
throw an error. Instead, just grab the first port and attach NAT to it.
That's not completely correct, so a follow up patch is forthcoming that
will add support to os-client-config for such a configuration value.
However, in the mean time, just grab one, because why not.
Change-Id: Ib3a49c0efc72a6de0bdd84c3fe1f34cc5f553a8c