rgw: role: use the list roles from role svc classes

rgw: role: use the list roles from role svc classes

This keeps the implementation of storage seperate from the RGWRole class. Also
drop the get_roles_by_path_prefix static function as this is currently
implemented in the backend and can be directly accessed from role ctl classes.

Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>
(cherry picked from commit 846a0e4ec96d2e06fe96984d7be20d16bbb80c8f)