Following-on from Iffb462371939989b03e5d6ac6c5df63aa7708513, instead
of directly referring to a hostname when adding the bastion host to
the inventory for the production playbooks, this finds it from the
first element of the "bastion" group.
As we do this twice for the run and post playbooks, abstract it into a
role.
The host value is currently "bridge.openstack.org" -- as is the
existing hard-coding -- thus this is intended to be a no-op change.
It is setting the foundation to make replacing the bastion host a
simpler process in the future.
Change-Id: I286796ebd71173019a627f8fe8d9a25d0bfc575a
51611845d4