Validate S3 tokens against Keystone

- Added config option to allow S3 to use Keystone auth
- Implemented JSONDecoder for KeystoneToken
- RGW_Auth_S3::authorize now uses rgw_store_user_info on keystone auth
- Minor fix in get_canon_resource; dout is now after the assignment

Reviewed-by: Yehuda Sadeh<yehuda@inktank.com>
Signed-off-by: Roald J. van Loon <roaldvanloon@gmail.com>