Fix joint_trajectory_controller execution - set stopped_velocity_tolerance to zero

[livanov93]

@MGBla Addresses #36

[MGBla]

It seems like the parameters are not added to the parameter service. There are only the following parameters:

/ur_joint_trajectory_controller:
  action_monitor_rate
  allow_partial_joints_goal
  constraints.goal_time
  constraints.stopped_velocity_tolerance
  joints
  state_publish_rate
  use_sim_time

As stated in #36 the root of the error was the stopped_velocity_tolerance.
But on a general note, I really don't think we should specify the tolerances in this yaml file. Since the JointTrajector msg specifies all the tolerance in its definition I think we would circumvent the intended mechanics here.