Add tf_prefix parameter for hardware interface

[fmauch]

When using a robot description that uses a tf_prefix for the robot,
the joint names will be different than expected in the hardware interface.
This commits ads a tf_prefix parameter to the hardware node in order to pass
a tf_prefix into the node.

Obviously, this could be solved differently, e.g. by reading the joint names from the parameter server completely, I've seen that approach in other RobotHW implementations, as well. That would have the benefit of being completely flexible with the downside of adding an additional list parameter to the node.

[fmauch]

Note, this is connected to husky/husky#183