overlay sample for groovy

When the groovy, the overlay_sample.py show the error.

```
Traceback (most recent call last):
  File "/home/***/ros/groovy/jsk-ros-pkg/jsk_visualization/jsk_rviz_plugins/scripts/overlay_sample.py", line 3, in <module>
    from jsk_rviz_plugins.msg import *
ImportError: No module named jsk_rviz_plugins.msg
```