Add support for custom hostname

This is a feature request to add support for setting up a custom hostname, so that rabbitmq-server can advertise its hostname in the relations to the other units instead of its IP address.

My use case is to deploy rabbitmq-server in the public cloud and relate it with the consuming units deployed on premise. The IP address of the resulting rabbimq-server unit is in private VPC and cannot be accessed directly by the consuming units.

Assigning the Elastic IP to the VM hosting rabbitmq-server won't help, since rabbitmq-server is not aware of this IP address.

The solution would be to create a record in Route53 zone, pointing to the Elastic IP attached to the VM. This makes it possible for consuming units to access rabbitmq-server. However, the consuming units must have an automatic way to learn about rabbitmq-server's hostname, ideally via relations.

I think the simple change would be to update the code in https://github.com/openstack/charm-rabbitmq-server/blob/master/hooks/rabbitmq_server_relations.py#L272 with the following logic:

If the custom hostname is defined in charm config:
  host_addr = hostname from the config
Else
  host_addr = ch_ip.get_relation_ip(...)