rgw: support more rgw admin API methods

[maksim-kharitonov]

add support of some useful methods of rgw admin API:

• get specific user's buckets with stats: get-bucket-info
• create/delete credentials: create-key remove-key
• individual bucket quota support set-quota-for-an-individual-bucket

[sebastianriese]

Another thing that could be tackled while one is at it, is to use /user?list to list users (this endpoint exists at least for newer versions of ceph), instead of using /metadata/user – or otherwise document the reason the latter is preferred for some arcane reason. Sadly, neither endpoint is documented.

Also related (in that it adds more rgw admin ops methods: #644 )