frontend: do not crash on missing output in output_for_cli

Do not crash in Command.output_for_cli when an output value is missing.

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

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

ipalib/frontend.py

@@ -1048,7 +1048,7 @@ class Command(HasParam):
             outp = self.output[o]
             if 'no_display' in outp.flags:
                 continue
-            result = output[o]
+            result = output.get(o)
 
             if o == 'value':
                 continue