CBQE-1150: distribute queries across active nodes

Select random node for each query.  Also fixes
cases when orchestrator id down or is rebalanced out.

Change-Id: Ic895cf2f56a55ea6a33a8a405b25faef9166f17b
Reviewed-on: http://review.couchbase.org/25273
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: Tommie McAfee <tommie@couchbase.com>