Observe failing when number of available nodes is less than the configured replica count

Observe fails when the amount of available nodes is less than the replica count, this happens when e.g. a bucket is configured with 4 replicas but the cluster only has two nodes.

Specifically, it attempts to access an array index of -1 when the vbucket config for the replica is not there. This obviously causes the client to raise an exception (about accessing an invalid index)

This is probably best explained by the changeset here

http://review.couchbase.org/21183