schema: do not validate unrequested params in command_defaults

Request specific params when getting the defaults instead of getting
defaults for all params and filtering the result.

This fixes command_defaults failing with validation errors on unrequested
params.

https://fedorahosted.org/freeipa/ticket/4739

Reviewed-By: David Kupka <dkupka@redhat.com>

ipaserver/plugins/schema.py

@@ -229,8 +229,7 @@ class command_defaults(PKQuery):
             raise errors.ConversionError(name=name,
                                          error=_("must be a dictionary"))
 
-        result = command.get_default(**kw)
-        result = {n: v for n, v in result.items() if n in params}
+        result = command.get_default(params, **kw)
 
         return dict(result=result)