Add time factor support for trajectory time parametrization

This adds support for setting a motion_execution_time_factor in the MotionPlanRequest msg, which can be used to scale the velocity bounds in add_time_parametrization.

Includes necessary changes to
* move_group
* move_group_interface
* Spinbox for motion planning rviz plugin

Requires other pulls requests in moveit_msgs and moveit_core which I'll add soon.