HA support for radosgw (WIP)

- mostly based on nova_dashboard

> But isn't the current behavior (when one cluster node has different additional roles) buggy? Why should we enforce same roles for cluster members?

Yes, it's buggy in general. But for some roles (ceph-mon, ceph-osd), there's no need to add HA and it's fine to have them hosted on cluster nodes because the software (ceph) is HA by default. So these roles would be allowed anyway.

In the end, this is really a edge case, but a edge case we need to protect against.