radosgw-admin: use check_status for task success

radosgw-admin: use check_status for task success

Alter the code to use the 'check_status=True'
option in rgwadmin() rather than following the
call with 'assert not err'. Should make the
tests a bit more clear and result in a more
useful error (throw the call stack rather than
just 'assert new err failed').

Signed-off-by: Joe Buck <jbbuck@gmail.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>