[sheepdog-users] Disable recovery when there's not enough space

[Valerio Pachera]

https://bugs.launchpad.net/sheepdog-project/+bug/1377402

Please, consider this as a critical bug.

I'm going to test how the cluster behaves if strict mode is enable in such case.
Anyway, it's a different approach.
The idea is to disable recovery and show a degraded state.
Strict mode would stop the cluster I guess.