Check if joy_node and joy_throttler nodes reliably connect

[mortenfyhn]

joystick_interface/launch/joystick_interface.launch launches three nodes:

• joy_node
• throttle
• joystick_interface

joy_node outputs on the topic joy. throttle reads that topic and outputs on a topic joy_throttle (which is default, it just uses <input_topic>_throttle unless otherwise specified). I've noticed that sometimes the nodes don't seem to connect, when inspecting the node graph with rqt_graph. Should check if this is still a problem, and if it is, fix it.