remove leading '/' from DEFAULT_PLANNING_SCENE_SERVICE

 otherwise planning_scene_rviz_plugin fails with 'Failed to call service /get_planning_scene ...' if move_group is in a namespace different from '/'
@shaun-edwards Given that MoveIt is < 1.0, an email to the users list letting people know about the change is probably a better move than adding extra backwards comparability code and deprecation warnings. There is no assurance of API compatibility when < 1.0, and we've already broken it far more than this over the past year or so.