This allows neutron service endpoints to use custom hostnames, and adds the
following variables:

* neutron_internal_fqdn
* neutron_external_fqdn

These default to the old values of kolla_internal_fqdn or
kolla_external_fqdn.

This also adds a neutron_server_listen_port option, which defaults to
neutron_server_port for backward compatibility.

This option allow the user to differentiate between the port the
service listens on, and the port the service is reachable on. This is
useful for external load balancers which live on the same host as the
service itself.

Change-Id: I87d7387326b6eaa6adae1600b48d480319d10676
Implements: blueprint service-hostnames