Get planning scene check

Fix for error that sometimes occurs when the Rviz Motion Planning plugin is launch with the move_group node in the same launch file. Before making a service call to get an updated planning scene, this causes it to wait for the service to exist up to 5 seconds before showing the error

```
Failed to call service /get_planning_scene
```

The 5 seconds is arbitrary but is enough to allow the timing issue to usually work out. This fix doesn't change any functionality, just helps remove the error message.