Merge pull request #6784 from ceph/wip-5073

rgw multitenancy

Buckets and users can now have a tenant associated with them. Buckets can be specified as [<tenant>:]<bucket> and users can be specified as [<tenant>$]<uid>. The existing users and buckets are part of the default global tenant.
This is first phase. Future work will add tighter integration (e.g., swift tenant mapping, s3 subdomain tenant mapping, etc.)

Reviewed-by: Pete Zaitcev <zaitcev@redhat.com>
Reviewed-by: Yehuda Sadeh <yehuda@redhat.com>