SITL despawn gazebo model on shutdown hook

[mhl787156]

Currently if the SITL stops or fails or needs respawning, both the SITL and Gazebo instance have to be restarted.

It would be nice if SITL needed to be restarted or stopped, that the Gazebo model is removed too.

This would allow a nice way of doing repeatable experimentation as restarting the SITL now causes a host of GPS and interal state errors as the drone doesn't start where the SITL thinks it should be.