rbd: add methods to set and get snapshot limits

rbd: add methods to set and get snapshot limits

Add a subcommand, rbd snap limit set <image> --limit <integer>, to
limit the number of snapshots that may be made of a given image.
To clear, use rbd snap limit clear <image>. Add an omap
key, 'snap_limit' to the RBD header for implementation. Add object
classes to set and query the limit. Older OSDs will ignore the limit.

Fixes: http://tracker.ceph.com/issues/15706
Signed-off-by: Douglas Fuller <dfuller@redhat.com>