avocado.core.app: Improve compatibility with older stevedore

Older versions of stevedore does not support `__contains__` method. Use
try/except to handle subcommand handling.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/core/app.py

@@ -68,8 +68,9 @@ class AvocadoApp(object):
                 view.notify(event='error', msg=msg)
 
     def run(self):
-        subcommand = self.parser.args.subcommand
-        if subcommand in self.cli_cmd_dispatcher:
-            extension = self.cli_cmd_dispatcher[subcommand]
-            method = extension.obj.run
-            return method(self.parser.args)
+        try:
+            extension = self.cli_cmd_dispatcher[self.parser.args.subcommand]
+        except KeyError:
+            return
+        method = extension.obj.run
+        return method(self.parser.args)