MB-11042 print correct usage help for cbstats

Shows the mandatory options and optional arguments
Also fixes cbvbucketctl, cbepctl and any user of clitool.py

Change-Id: I554f39d295d16b19d5cbf675c43c142f43ba2f7d
Reviewed-on: http://review.couchbase.org/40069
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>