Improve OpenStack integration and multitenancy docs for radosgw

Improve OpenStack integration and multitenancy docs for radosgw

Add explanatory information on:

* "rgw swift account in url" (including the Swift account in the Swift
  API url and Keystone endpoint)
* "rgw swift versioning enabled" (enabling Swift object versioning)
* "rgw s3 auth use keystone" (enabling S3 authentication against
  Keystone)
* "rgw keystone implicit tenants" (multi-tenancy via Keystone, including
  its implications for the Swift and S3 APIs)

Fixes: http://tracker.ceph.com/issues/36765
Signed-off-by: Florian Haas <florian@citynetwork.eu>