fix: fix dual panda motion generator PID gains bug

[rickstaa]

@marcbone This pull request makes sure that the 'arm_id' is taken into account when the motion generator PID gains are loaded onto the ROS parameter server. I noticed that we forgot to include the dual panda setup described by @rhaschke in #196 (comment) when we merged 2514ae1.

How to reproduce.

1. Build the franka_ros package.
2. Source the franka_ros package.
3. Try to spawn two panda robots using the following commands in different terminals:

# Start single Gazebo instance (with empty world):
roslaunch gazebo_ros empty_world.launch world_name:=worlds/empty.world use_sim_time:=true paused:=true
# Spawn robot panda1 into namespace left
ROS_NAMESPACE=left roslaunch franka_gazebo panda.launch gazebo:=false arm_id:=panda1 y:=0 paused:=true
# Spawn robot panda2 into namespace right
ROS_NAMESPACE=right roslaunch franka_gazebo panda.launch gazebo:=false arm_id:=panda2 y:=1 paused:=false

4. Be greeted by the following errors:

[/gazebo] [ERROR] [WallTime: 1647879761.328384282]: No p gain specified for pid.  Namespace: /left/motion_generators/position/gains/panda1_joint1
[/gazebo] [ INFO] [WallTime: 1647879761.328639773]: Found transmission interface of joint 'panda1_joint2': hardware_interface/PositionJointInterface
[/gazebo] [ERROR] [WallTime: 1647879761.328962427]: No p gain specified for pid.  Namespace: /left/motion_generators/position/gains/panda1_joint2
[/gazebo] [ INFO] [WallTime: 1647879761.329020354]: Found transmission interface of joint 'panda1_joint3': hardware_interface/PositionJointInterface
[/gazebo] [ERROR] [WallTime: 1647879761.329330334]: No p gain specified for pid.  Namespace: /left/motion_generators/position/gains/panda1_joint3
[/gazebo] [ INFO] [WallTime: 1647879761.329358906]: Found transmission interface of joint 'panda1_joint4': hardware_interface/PositionJointInterface
[/gazebo] [ERROR] [WallTime: 1647879761.329677448]: No p gain specified for pid.  Namespace: /left/motion_generators/position/gains/panda1_joint4
[/gazebo] [ INFO] [WallTime: 1647879761.329712635]: Found transmission interface of joint 'panda1_joint5': hardware_interface/PositionJointInterface
[/gazebo] [ERROR] [WallTime: 1647879761.329995357]: No p gain specified for pid.  Namespace: /left/motion_generators/position/gains/panda1_joint5
[/gazebo] [ INFO] [WallTime: 1647879761.330029905]: Found transmission interface of joint 'panda1_joint6': hardware_interface/PositionJointInterface
[/gazebo] [ERROR] [WallTime: 1647879761.330305189]: No p gain specified for pid.  Namespace: /left/motion_generators/position/gains/panda1_joint6
[/gazebo] [ INFO] [WallTime: 1647879761.330339015]: Found transmission interface of joint 'panda1_joint7': hardware_interface/PositionJointInterface
[/gazebo] [ERROR] [WallTime: 1647879761.330564114]: No p gain specified for pid.  Namespace: /left/motion_generators/position/gains/panda1_joint7
[/gazebo] [ INFO] [WallTime: 1647879761.330595433]: Found transmission interface of joint 'panda1_joint1': hardware_interface/VelocityJointInterface
[/gazebo] [ERROR] [WallTime: 1647879761.330834847]: No p gain specified for pid.  Namespace: /left/motion_generators/velocity/gains/panda1_joint1
[/gazebo] [ INFO] [WallTime: 1647879761.330882767]: Found transmission interface of joint 'panda1_joint2': hardware_interface/VelocityJointInterface
[/gazebo] [ERROR] [WallTime: 1647879761.331115950]: No p gain specified for pid.  Namespace: /left/motion_generators/velocity/gains/panda1_joint2
[/gazebo] [ INFO] [WallTime: 1647879761.331144046]: Found transmission interface of joint 'panda1_joint3': hardware_interface/VelocityJointInterface
[/gazebo] [ERROR] [WallTime: 1647879761.331350666]: No p gain specified for pid.  Namespace: /left/motion_generators/velocity/gains/panda1_joint3
[/gazebo] [ INFO] [WallTime: 1647879761.331378690]: Found transmission interface of joint 'panda1_joint4': hardware_interface/VelocityJointInterface
[/gazebo] [ERROR] [WallTime: 1647879761.331629974]: No p gain specified for pid.  Namespace: /left/motion_generators/velocity/gains/panda1_joint4
[/gazebo] [ INFO] [WallTime: 1647879761.331657665]: Found transmission interface of joint 'panda1_joint5': hardware_interface/VelocityJointInterface
[/gazebo] [ERROR] [WallTime: 1647879761.331887226]: No p gain specified for pid.  Namespace: /left/motion_generators/velocity/gains/panda1_joint5
[/gazebo] [ INFO] [WallTime: 1647879761.331916082]: Found transmission interface of joint 'panda1_joint6': hardware_interface/VelocityJointInterface
[/gazebo] [ERROR] [WallTime: 1647879761.332112102]: No p gain specified for pid.  Namespace: /left/motion_generators/velocity/gains/panda1_joint6
[/gazebo] [ INFO] [WallTime: 1647879761.332139902]: Found transmission interface of joint 'panda1_joint7': hardware_interface/VelocityJointInterface
[/gazebo] [ERROR] [WallTime: 1647879761.332333453]: No p gain specified for pid.  Namespace: /left/motion_generators/velocity/gains/panda1_joint7

[gollth]

Thanks for this @rickstaa, definitely missed that. We have a look and give you feedback shortly. Don't worry about the conflict, we'll take care of that (since we might slightly change the CHANGELOG line)