WIP: add a membershipchange test

[jcorbin]

Note even partially done, just to record the ongoing lack of such a test.

Depends on #114 so that we can add more nodes (towards triggering a real membership change).

TODO:

• [ ] make it so that we can destroy the steve exit nodes and then wait for ringpop reconverge
• [ ] or sort out some other reliable (non-flappy way) to guarantee a hard membership change