Add teleop parameter for server launch file

[knmcguire]

Add a teleop parameter that enables the joy node. In reality this is no longer used that often though, so that is why the default is off.

fixes #67

[whoenig]

I'd prefer the default to be true. We mostly use it for the "emergency" button. This condition should also be added to the teleop node itself, not only to joy?

[knmcguire]

Ahh oke then let's keep it to True then as default.

And yes you are right, I've added the condition to teleop as well, forgot that part!