CBQE-1226: fix http-ping return None for non-cluster nodes

check pools instead of nodes/self api to detect if node is still in a cluster

Change-Id: Id9c05d2f5a6ff4382bb12c8a13f4af012c31951c
Reviewed-on: http://review.couchbase.org/25738
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: Tommie McAfee <tommie@couchbase.com>