[Indigo] introduce launchfile argument for -J option of spawn_model

This PR adds a new optional launch file argument for ```cob_bringup_sim robot.launch``` that allows to set joint to initial configs other than 0.0.
The argument can than be used like this:
```
roslaunch cob_bringup_sim robot.launch robot:=raw3-1 robot_env:=empty initial_config:='-J arm_shoulder_pan_joint 0.0 -J arm_shoulder_lift_joint -1.5707 -J arm_elbow_joint 0.0 -J arm_wrist_1_joint -1.5707 -J arm_wrist_2_joint 0.0 -J arm_wrist_3_joint 0.0'
```


(This feature is currently being re-added to ```gazebo_ros_pkgs```....thus please hold this PR for a little longer...)