Annex to wip-4313

This branch adds two commits that may or may not be incorporated into the tree:

- A (rather ugly) python script that will perform tests on the 'config-key' monitor api, and
- A patch to the 'ceph' tool so it returns the appropriate error code as returned from 'do_command()' -- prior to this patch, all errors from 'do_command()' would be conflated into '1', making error handling from a script rather difficult.

The python script heavily depends on the second patch.  However, I am not sure if this is a change we want to push out, as scripts may already rely on this weird way of doing business -- teuthology, maybe? -- and I wouldn't want to break things for everybody.

If it happens not to be a convenient time to merge the 'ceph' tool patch (specially with Dan's management API coming out in the near future), none of this branch should me merged.