ceph CLI: valid() no longer returns bool, but just exception

ceph CLI: valid() no longer returns bool, but just exception

The type validation's valid() method was using a combination of
return code and exception to really indicate the same thing;
simplify by only raising on validation error, and change callers
to cope.  validate_one() follows suit.

Also, allow validate() to be called with args that are dicts
(for REST support) rather than bare words.  Rules: 'name':'value'
must both match descriptor's name and validate (through valid() for
the value.  If value is '', it's assumed to be the same as name,
(one can pass, for example, "detail" as one parameter to
REST, but it will still show up as {'detail':''} here).

Tweak validate()'s algorithm a bit in the process, and make
validate_command() exit the bestcmds loop immediately on first
full validation.

Signed-off-by: Dan Mick <dan.mick@inktank.com>