Fix "too few args validate"

Fix "too few args validate"

Check that number of validated arguments matches the number of required
arguments in the signature.  Also, sort all possible matches by
length of signature.  This way "ceph osd crush set" and
"ceph osd crush set <args>" can work while still insisting that
extra args or too few args are errors.

Also, restructure and factor out some of the work of validate() to make
its inner loop smaller and hopefully more comprehensible.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>