Fix generate keystone token from credentials

Request to v2.0/tokens must be a POST request and the request
should specify JSON in the content-type header and fix request
body JSON generation to have the right JSON format.

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>
Signed-off-by: Mouad Benchchaoui <m.benchchaoui@x-ion.de>
rgw: minor cleanup

Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>