fix: fix dual panda motion generator PID gains not found bug

This commit makes sure that the 'arm_id' is taken into account when the
motion generator PID gains are loaded onto the ROS parameter server.

franka_gazebo/config/franka_hw_sim.yaml

@@ -16,19 +16,19 @@ franka_gripper:
 motion_generators:
   position:
     gains:
-      panda_joint1: { p: 600, d: 30, i: 0 }
-      panda_joint2: { p: 600, d: 30, i: 0 }
-      panda_joint3: { p: 600, d: 30, i: 0 }
-      panda_joint4: { p: 600, d: 30, i: 0 }
-      panda_joint5: { p: 250, d: 10, i: 0 }
-      panda_joint6: { p: 150, d: 10, i: 0 }
-      panda_joint7: { p:  50, d:  5, i: 0 }
+      $(arg arm_id)_joint1: { p: 600, d: 30, i: 0 }
+      $(arg arm_id)_joint2: { p: 600, d: 30, i: 0 }
+      $(arg arm_id)_joint3: { p: 600, d: 30, i: 0 }
+      $(arg arm_id)_joint4: { p: 600, d: 30, i: 0 }
+      $(arg arm_id)_joint5: { p: 250, d: 10, i: 0 }
+      $(arg arm_id)_joint6: { p: 150, d: 10, i: 0 }
+      $(arg arm_id)_joint7: { p:  50, d:  5, i: 0 }
   velocity:
     gains:
-      panda_joint1: { p: 30, d: 0, i: 0 }
-      panda_joint2: { p: 30, d: 0, i: 0 }
-      panda_joint3: { p: 30, d: 0, i: 0 }
-      panda_joint4: { p: 30, d: 0, i: 0 }
-      panda_joint5: { p: 10, d: 0, i: 0 }
-      panda_joint6: { p: 10, d: 0, i: 0 }
-      panda_joint7: { p:  5, d:  0, i: 0 }
+      $(arg arm_id)_joint1: { p: 30, d: 0, i: 0 }
+      $(arg arm_id)_joint2: { p: 30, d: 0, i: 0 }
+      $(arg arm_id)_joint3: { p: 30, d: 0, i: 0 }
+      $(arg arm_id)_joint4: { p: 30, d: 0, i: 0 }
+      $(arg arm_id)_joint5: { p: 10, d: 0, i: 0 }
+      $(arg arm_id)_joint6: { p: 10, d: 0, i: 0 }
+      $(arg arm_id)_joint7: { p:  5, d:  0, i: 0 }