rbd.py: make Image a context manager
rbd.py: make Image a context manager

Signed-off-by: Josh Durgin <51652c783f70f5b272ebd02f1fd20d3af7a2449f@dreamhost.com>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph