Add support for Ceph RadosGW integration

* Register Swift-compatible endpoints in Keystone
* Load balance across RadosGW API servers using HAProxy

The support is exercised in the cephadm CI jobs, but since RGW is
not currently enabled via cephadm, it is not yet tested.

https://docs.ceph.com/en/latest/radosgw/keystone/

Implements: blueprint ceph-rgw

Change-Id: I891c3ed4ed93512607afe65a42dd99596fd4dbf9