Add option to autofailover ephemeral bucket with 0 replicas

For pure cache use cases, it may be desirable to have 0 replicas.  However, this means that auto-failover would not normally kick in so that a node (or group) down results in unavailability for those vbuckets and requires manual intervention.

The user is requesting an option to allow auto-failover for ephemeral buckets with 0 replicas which would then go hand-in-hand with MB-30154 to auto-rebalance after that auto-failover.