Robot Loader: use symbolic names

Signed-off-by: NCleber Rosa <crosa@redhat.com>

optional_plugins/robot/avocado_robot/__init__.py

@@ -76,7 +76,7 @@ class RobotLoader(loader.TestLoader):
     def __init__(self, args, extra_params):
         super(RobotLoader, self).__init__(args, extra_params)
 
-    def discover(self, url, which_tests=False):
+    def discover(self, url, which_tests=loader.DEFAULT):
         avocado_suite = []
         subtests_filter = None