MB-11454: Add an option to output json in cbstats

Change-Id: I0adcb16adde82ddf3671e4edc0118061d684f2d4
Reviewed-on: http://review.couchbase.org/42084
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>