Default RABBIT_HOST to be the SERVICE_HOST.

Using localhost for RABBIT_HOST can cause problems - particularly
on systems where IPv6 is partially broken and localhost resolves
to an IPv6 address.

The service should logically be running on SERVICE_HOST unless
otherwise overridden.

Change-Id: I6efb71b8b95174770066f38403d5d428ec88c3b6
Merge "Default RABBIT_HOST to be the SERVICE_HOST."
Merge branch 'master' of github.com:openstack-dev/devstack