upgrade: dumpling-next -> dumpling-x

Make the final upgrade step unspecified so that it can be specified at
schedule time.  This let us test dumpling -> firefly, dumpling->emperor, or
whatever else we want.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #7 from ceph/wip-upgrade

upgrade: dumpling-next -> dumpling-x