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 '/'
From my understanding of the code, the node handle, `nh_` is initialized as follows `nh_('~')`.  This results in a private namespace.

If only the service name were changes, and the service client initialization were kept the same, then following difference would be observed:
  * *Before*: /<service_name>
  * *After*:  /<namespace>/<node_name>/<service_name>

It's not clear whether the service is meant to be private (this is not common), but to ensure this change doesn't break anything, the new service initialization is required.  Here are the two possible
  * *Global namespace (backwards compatible)*: /<service_name>
  * *Named namespace (not backwards compatible)*: /<namespace>/<service_name>

It's worth pointing out that this change **WILL** break anybody who has namespaced their MoveIt code and then worked around this issue.  One suggestion is to also create a global namespaced service to ensure backwards compatibility.  We could include a `deprecated` warning when the global service is called.