rgw, rgw-admin: add realm/zonegroup/zone id configurables

rgw, rgw-admin: add realm/zonegroup/zone id configurables

Add the following configurables:
 - rgw_realm_id
 - rgw_zonegroup_id
 - rgw_zone_id

radosgw-admin already had --realm-id, --zonegroup-id, --zone-id config options,
however, there were no such config options for radosgw. Adding these new config
options so that we could start radosgw with specifying the appropriate entity id
(vs. the entity names that was required previously). These also apply for
radosgw-admin.
The old radosgw-admin params are still valid for backward compatibility.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>