Allow user to choose mixed_position_velocity_controllers for Gazebo

[toliver]

We recently switched the default controllers for the gazebo hand to position controllers (instead of the previous mixed_position_velocity_controllers).

This is OK and is how we want it, but we should keep a separate yaml file for each of the controllers. Currently we have a single one:

https://github.com/shadow-robot/sr-ros-interface/blob/hydro-devel/sr_description/hand/config/hand_controller_gazebo.yaml

(we need to create a hand_controller_gazebo_mixed.yaml)

Then we should provide an argument on the launch files to decide which controllers should be used:

https://github.com/shadow-robot/sr-ros-interface/blob/hydro-devel/sr_hand/launch/gazebo/gazebo_hand.launch https://github.com/shadow-robot/sr-ros-interface/blob/hydro-devel/sr_hand/launch/gazebo/gazebo_arm_and_hand.launch

https://github.com/shadow-robot/sr-ros-interface/blob/hydro-devel/sr_hand/launch/gazebo/loaders/hand_controllers.launch

[markpitchless]

I've been thinking we could maybe change the way we do this a little. We can load all of the config for all available controllers onto the param server and then load (but not start) them all in the controller manager. Then just start the one set (position) by default. Changing controllers is then just a start/stop of the different sets of controllers, no loading and unloading. The controller manager can then handily show a list of all available controllers.

[guihomework]

https://github.com/shadow-robot/sr-ros-interface/pull/147 adds a hand_mixed_controller_gazebo.yaml but no way to load it or switch it. Our launch file pointing to it is not in sr_hand but in our own packages. Would be nice to get a generic way to load different controllers in sim as on the real hand.