Working with gazebo

[tahsinkose]

This makes the necessary changes for a working Gazebo integration. I've specified effort_joint_trajectory_controller as an example, but it could be position or velocity as well.

[rhaschke]

franka_ros provides different default controllers for real and sim. Thus, we need to offer an argument to choose the joint trajectory controller type. I implemented this in c38a59a. Closing here.
@gollth: This PR here is exactly what I was afraid of in frankaemika/franka_ros#186 (comment): users will try to hard-code the controller type and they need to change between sim and real.

[gollth]

franka_ros provides different default controllers for real and sim

There is no "default" controller neither for real nor for sim in franka_ros. Both offer the effort_joint_trajectory_interface which you could use as "default" if you will.