rbd: support plain/json/xml output formatting

rbd: support plain/json/xml output formatting

This patch renames the --format option to --image-format, for
specifying the RBD image format, and uses --format to specify the
output formatting (to be consistent with the other ceph tools). To
avoid breaking backwards compatibility with existing scripts, rbd will
still accept --format [1|2] for the image format, but will print a
warning message, noting its use is deprecated.

The rbd subcommands that support the new --format option are : ls, info, snap
list, children, showmapped, lock list.

Signed-off-by: Stratos Psomadakis <psomas@grnet.gr>
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>