rgw/role: add rgwrados::role interface for role metadata

rgw/role: add rgwrados::role interface for role metadata

move everything from class RadosRole into driver/rados/role.cc and use
RGWSI_SysObj instead of the metadata backend. narrows the RGWRole
interface by handling the name/path objects internally

rgwrados::role::write() adds/remove name and path linkages in case they
change. admin ops don't allow this, but 'metadata put' could upload
arbitrary json that does change them

Signed-off-by: Casey Bodley <cbodley@redhat.com>