Segfault using the gazebo_ros_openni_kinect plugin

The current version of the gazebo_ros_openni_kinect plugin in gazebo_ros_pkg uses the this->rosnode_ pointer to advertise point clouds and depth images in its `GazeboRosOpenniKinect::Load` method ([line 110](https://github.com/osrf/gazebo_ros_pkgs/blob/hydro-devel/gazebo_plugins/src/gazebo_ros_openni_kinect.cpp#L110)), but this pointer might not have been initialized yet as initialization happens in a separate thread in `GazeboRosCameraUtils::LoadThread` ([line 259](https://github.com/osrf/gazebo_ros_pkgs/blob/hydro-devel/gazebo_plugins/src/gazebo_ros_camera_utils.cpp#L259)) without synchronization.