CBST-12: adding --cluster arg to push_stats tool

for instance, we can't assume dbs in seriesly have 'default' prefix when collecting stats as the prefix
will actually match whatever was set CB_CLUSTER_TAG of the testcfg.py file. so adding --cluster option for other such cases.

Change-Id: Id21be47c5cb45ad6874252e2425982fa609bedb0
Reviewed-on: http://review.couchbase.org/30451
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: Tommie McAfee <tommie@couchbase.com>