Allow different MoveGroup namespace

[JStech]

The MoveGroup interface uses the default (empty) namespace, and so it won't work with a robot that is using a non-empty namespace.

[playertr]

In my case, I was able to work around this by starting rviz in my robot's namespace. Previously, the calls to "robot_description" and "planning_scene_monitor" within handeye_control_widget.cpp and handeye_context_widget.cpp had been attempting to access these topics relative to the rviz namespace. By changing the namespace rviz was launched in, the calibration plugin can access these values without significant modification of the rest of my launch file.