Notifier's default_publisher_id option requires a 'host' option to be registered

This review:

  https://review.openstack.org/14466

threw up an interesting issue. In openstack.common.notifier.api we do:

    cfg.StrOpt('default_publisher_id',
               default='$host',
               help='Default publisher_id for outgoing notifications'),

We also have this:

  def publisher_id(service, host=None):
      if not host:
          host = CONF.host
      return "%s.%s" % (service, host)

which basically means projects can only use this API if they first register a 'host' configuration option.

I guess we have a few options:

1) We could make 'host' a config option registered by some base part of the library - I'm a little reluctant to do this because the semantics are a little weird. What does changing it mean? It affects a whole bunch of places in Nova, for example. Also, what part of openstack-common would it make sense to register the option from?

2) Make the default for default_publisher_id be socket.getfqdn() - this sucks because the behaviour of 'just change the host option' breaks

3) We make projects which use the notifier API supply the publisher ID in a setup function - I think this is something we want to support anyway, so perhaps doing this makes most sense