webadmin: Added overload for Frontend.runMultipleAction()

This overload doesn't allows to pass a callback, but doesn't force to
pass state (where often the one that does is invoked with null).

Change-Id: I18f4cf8b5a1fac4ea7bcf41c88a8ada55b81705d
Signed-off-by: Lior Vernia <lvernia@redhat.com>