Hide the -a argument in cbstats

The -a argument is used by cbcollectinfo so we can't just
get rid of it.

Change-Id: I0f04c2d454c117271b44176df904e90a938965e9
Reviewed-on: http://review.couchbase.org/42297
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>