CBQE-1422 postcondition check for ep-engine stats for whole cluster

The postcondition format will be something like:
 "workload":[{"spec": "s:3,u:22,g:70,d:3,e:2,m:5,ttl:3000,coq:defaultph1keys,ccq:defaultph2keys,ops:15000",
             "conditions" : {"post": {"conditions": "ep_warmup_thread = complete", "cluster_check"="True"}}},
 {"spec": "b:saslbucket,pwd:password,s:3,u:22,g:70,d:3,e:2,m:5,ttl:3000,coq:saslph1keys,ccq:saslph2keys,ops:15000",
              "conditions" : {"post": {"conditions": "ep_warmup_thread = complete", "cluster_check"="True"}}}

Therefore we support both one node stats check (for example, when restart one node)
and per-node stats check (for example when restart whole cluster). Even when
cfg.COUCHBASE_IP is removed from cluster. This still works

Change-Id: I8d33e24e54dd7f72a70a6d56e71aba37d2d79a03
Reviewed-on: http://review.couchbase.org/27646
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>