Set robot description parameter for controllers

[annaborn]

Hello,

this patch sets the robot description parameter, so it can be accessible from controllers.

Similar to the patch done for gazebo_ros2_control ros-controls/gazebo_ros2_control#277

Without the patch:

/controller_manager:
  joint_state_broadcaster.type
  qos_overrides./clock.subscription.depth
  qos_overrides./clock.subscription.durability
  qos_overrides./clock.subscription.history
  qos_overrides./clock.subscription.reliability
  qos_overrides./parameter_events.publisher.depth
  qos_overrides./parameter_events.publisher.durability
  qos_overrides./parameter_events.publisher.history
  qos_overrides./parameter_events.publisher.reliability
  update_rate
  use_sim_time
/joint_state_broadcaster:
  extra_joints
  interfaces
  joints
  map_interface_to_joint_state.effort
  map_interface_to_joint_state.position
  map_interface_to_joint_state.velocity
  qos_overrides./clock.subscription.depth
  qos_overrides./clock.subscription.durability
  qos_overrides./clock.subscription.history
  qos_overrides./clock.subscription.reliability
  update_rate
  use_local_topics
  use_sim_time

With the patch:

/controller_manager:
  joint_state_broadcaster.type
  qos_overrides./clock.subscription.depth
  qos_overrides./clock.subscription.durability
  qos_overrides./clock.subscription.history
  qos_overrides./clock.subscription.reliability
  qos_overrides./parameter_events.publisher.depth
  qos_overrides./parameter_events.publisher.durability
  qos_overrides./parameter_events.publisher.history
  qos_overrides./parameter_events.publisher.reliability
  robot_description
  update_rate
  use_sim_time
/joint_state_broadcaster:
  extra_joints
  interfaces
  joints
  map_interface_to_joint_state.effort
  map_interface_to_joint_state.position
  map_interface_to_joint_state.velocity
  qos_overrides./clock.subscription.depth
  qos_overrides./clock.subscription.durability
  qos_overrides./clock.subscription.history
  qos_overrides./clock.subscription.reliability
  robot_description
  update_rate
  use_local_topics
  use_sim_time

It will fix the open issue: #168

@ahcorde
@christophfroehlich