avocado.cli.app: Display --help output on empty args list

Instead of displaying a not very helpful 'too few args'
message, let's display the full --help output on an
empty args list. Comes with a functional test.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

avocado/cli/app.py

@@ -17,6 +17,7 @@ The core Avocado application.
 """
 
 import os
+import sys
 
 from argparse import ArgumentParser
 
@@ -64,7 +65,12 @@ class AvocadoApp(object):
             description='valid subcommands',
             help='subcommand help')
         self.load_plugin_manager(args.plugins_dir)
-        args, _ = self.app_parser.parse_known_args()
+
+        default_args = None
+        if not sys.argv[1:]:
+            default_args = ['--help']
+        args, _ = self.app_parser.parse_known_args(args=default_args)
+
         self.plugin_manager.activate(args)
         self.args = self.app_parser.parse_args()
 

selftests/all/functional/avocado/basic_tests.py

@@ -154,6 +154,15 @@ class RunnerOperationTest(unittest.TestCase):
         self.assertEqual(result.exit_status, expected_rc)
         self.assertEqual(result.stderr, expected_output)
 
+    def test_empty_args_list(self):
+        os.chdir(basedir)
+        cmd_line = './scripts/avocado'
+        result = process.run(cmd_line, ignore_status=True)
+        expected_rc = 0
+        unexpected_output = 'too few arguments'
+        self.assertEqual(result.exit_status, expected_rc)
+        self.assertNotIn(unexpected_output, result.stdout)
+
     def test_empty_test_list(self):
         os.chdir(basedir)
         cmd_line = './scripts/avocado run'