avocado.plugins.runner: Introduce --multiplex-file option

With this, people will be able to introduce a --multiplex-file
(short -m) option to the run subcommand, that provides the
path to a multiplex file, that will be turned into a test
matrix (list of dicts) that will be run by the test runner.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

avocado/plugins/runner.py

@@ -82,7 +82,12 @@ class TestRunner(plugin.Plugin):
         myparser.add_argument('url', type=str,
                               help=('Test module names or paths to dropin tests '
                                     '(space separated)'),
-                              nargs='?', default='')
+                              nargs='?', default=None)
+
+        myparser.add_argument('-m', '--multiplex-file', type=str,
+                              help=('Path to an avocado multiplex '
+                                    '(.mplex) file '),
+                              nargs='?', default=None)
 
         myparser.add_argument('--keep-tmp-files', action='store_true',
                               help=('Keep temporary files generated by tests. '