Teleop interface for applying force/torque to models

[bsb808]

Desired Capability

Interface joy teleop to the force/torque interface in gazebo. The proximate need is to be able to test the features of the electrical mating plugins - see https://github.com/Field-Robotics-Lab/dave/wiki/Electrical-Plug-Mating-Plugin and #189. Currently testing these features requires a lot of extra stuff: a robot, feedback control of the robot, manipulator and manipulator control. It would be helpful and improve our testing if instead we could directly "drive" the plug into the socket.

This tool may exist - so first step is to make sure we aren't reinventing the wheel.

[mabelzhang]

Can be used for #206