Wip 7810

NB this is based off https://github.com/ceph/teuthology/pull/298
Ah, right. I'm sorry, I can't always keep all these PRs straight.

I'm really not sure. I still don't really get *why* we are 'resolving roles into concrete roles'. Maybe this is something that other tasks would love to use - maybe it's irrelevant. I feel a little dumb, but I just don't really get it.