Allow nova services to use independent hostnames

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

* nova_internal_fqdn
* nova_external_fqdn
* placement_internal_fqdn
* placement_external_fqdn
* nova_novncproxy_fqdn
* nova_spicehtml5proxy_fqdn
* nova_serialproxy_fqdn

These default to the old values of kolla_internal_fqdn or
kolla_external_fqdn.

This also adds the following variables:

* nova_api_listen_port
* nova_metadata_listen_port
* nova_novncproxy_listen_port
* nova_spicehtml5proxy_listen_port
* nova_serialproxy_listen_port
* placement_api_listen_port

These default to <service>_port, e.g. nova_api_port, for backward
compatibility.

These options 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: I7bcce56a2138eeadcabac79dd07c8dba1c5af644
Implements: blueprint service-hostnames