Wip 7810

NB this is based off https://github.com/ceph/teuthology/pull/298
I am sorely tempted to roll this back into the ceph task at this point, as is the only place it is used.  It makes a lot more sense with context.  I was moving it here in part because you had asked me to in https://github.com/ceph/teuthology/pull/298 -- do you still feel that this belongs here?