"fixed" GetReplica test

The GetReplica tests failed for "misconfigured" clusters with
a higher number of replicas than number of nodes.

Change-Id: I4f0b1e882a6098bc13f0c3f0d4841f1ff9b2a2f8
Reviewed-on: http://review.couchbase.org/25433
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>