Rollback functionality

[anthturner]

We pre-test and expect providers to be as honest as possible in that test, but things may still fail. We can't guarantee permissions for some things unless we try and fail, so there's not a good way to necessarily test every variable.

To that end, we should try to include a "rollback" feature in the future to avoid severe failures if something goes wrong mid-cycle.

[drub0y]

Yeah, I was going to write something up on how the entire rekeying workflow has the potential to fail (in a couple ways) and leave things in an inconsistent state where things are potentially broken. I think a big scan of the implementation needs to happen to identify all the possible inconsistent states things could end up in and then have a discussion how to mitigate.

[anthturner]

Feature infrastructure is complete as of #101 but individual Providers don't have rollback logic yet.