Add stats failovers to cbstats script

Change-Id: Ie593ff2e32c250b5fd82f764ba60c03d1ae0d499
Reviewed-on: http://review.couchbase.org/32670
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>