Kobuki Gazebo Updates.

It utilise [GazeboRos Utilities](https://github.com/ros-simulation/gazebo_ros_pkgs/blob/hydro-devel/gazebo_plugins/include/gazebo_plugins/gazebo_ros_utils.h) for tf prefix and topic namespacing.

Referenced [GazeboRosDiffDrive](https://github.com/ros-simulation/gazebo_ros_pkgs/blob/hydro-devel/gazebo_plugins/src/gazebo_ros_diff_drive.cpp).

Also now it has `base_prefix` parameter to configure mobile base namespace. So now we can use topics in following way.

odom
tf
joint_state
<base_prefix>/commands/velocity
<base_prefix>/sensors/xxx
<base_prefix>/events/xxx
GitHub makes this look confusing. It looks like you added this spinOnce here, but it's actually already there.

Anyway, having to spinOnce here looks wrong to me. Even this spin() here misses the rate. I wonder, if it is used at all. This might be a left over from long time back, e.g. further up you find hints that a separate "ros_spinner_thread" once was used. We can drop this spin() function here completely.