motion_planning_rviz_plugin: add move_group namespace option

This allows multiple motion_planning_rviz_plugin /
planning_scene_rviz_plugin to be used in RViz and connect to
differently-namespaced move_group nodes.
Merge pull request #475 from jhu-lcsr-forks/rviz-move-group-namespace-option

motion_planning_rviz_plugin: add move_group namespace option