Prevent Graceful Failover in the event of disk issues

The graceful fail-over functionality shouldn't be used in the situation where there is a disk failure.

Both the CLI and UI should prevent triggering a graceful failover if the disk is unavailable.

Couchbase Server has periodic snapshots that are used for keeping track of multiple revisions of documents and roll-backs, these are written down to disk. The graceful fail-over function relies on being able to read these snapshots from disk from the node being failed-over to function correctly. When there is a disk failure, a hard failover is the only option.