CBQE-928: wait_for_ns_servers befor upgrade for warmup cases

Change-Id: Ic3c0566199907131670f7ffba2547f043f35c827
Reviewed-on: http://review.couchbase.org/23820
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>