Stop running shells when closing host handler.

        # running shells

        self.shells = dict()

    def stop(self):

        # kill all currently running shells

        for shell in self.shells.values():

            shell.kill()

            shell.close_cb()

        self.shells.clear()

        BasePlugin.stop(self)

    def execute_command(self, command):

        """Execute an arbitrary shell command on the host.