rgw: pre-generate bucket object version before creating it

Use this version when creating the bucket instead of letting the
objclass auto-generate it, so that we could return it to the slave
region when creating its bucket on the master.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: add system users

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: clean up s3 signing code

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