Add support for rabbitmq using the certificates relation
to secure traffic with clients. Support for users supplying
their own certs and for rabbit acting as its own Certificate
Authority should be unchanged. The follwoing additional changes
were needed:
* Charm helper sync
* Update rabbit to use get_relation_ip from charm helpers which
is the standard helper for calculating endpoint ip addresses.
* Create network helpers file to avoid circular dependancies.
Change-Id: Ie60893e660efe1f8b0a0d42aaaecfbd9aae6f97c