We should namespace our parameters the same (under 'constraints/' for the node parameters) namespace and the 'stopped_velocity_tolerance' parameters, the latter of which seems interesting for implementation specification.  They describe it ...
here http://wiki.ros.org/joint_trajectory_controller
 as "Velocity to be considered approximately equal to zero"
and here http://wiki.ros.org/joint_trajectory_action
as "The maximum velocity at the end of the trajectory for the joint to be considered stopped. Given in units of joint position (generally radians for revolute joints or meters for prismatic joints). If this constraint is satisfied, the goal is successful.":