This commit deals with renaming user. Under the hood, the attributes of an

This commit deals with renaming user. Under the hood, the attributes of an
existing user are moved under the new user namespace specified by "--new-uid".
It calls bucket link and bucket chown to link the buckets and objects to the new
user namespace. Access and secret keys of the user(and the subusers) are preserved.

Usage: "radosgw-admin user rename --uid=<> --new-uid=<>"

Signed-off-by: Shilpa Jagannath <smanjara@redhat.com>