Error if user omits attributes for options

If user indicates manipulation of
attributes through -c, -p, or -e,
but none are specified, print an error
instead.

confluent_client/bin/nodeattrib

@@ -105,7 +105,10 @@ if len(args) > 1:
             requestargs=args[1:]
     except:
         pass
-
+elif options.clear or options.environment or options.prompt:
+    sys.stderr.write('Attribute names required with specified options\n')
+    argparser.print_help()
+    exitcode = 400
 if exitcode != 0:
     sys.exit(exitcode)