rgw: create conf file hierarchy

Fixes: #5978

Create a more flexible conf file hierarchy.  E.g., for x.y.z
we'd go [x.y.z] -> [x.y] -> [x] -> global

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

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