SIMULATION: navigator does not spawn properly

[kev-the-dev]

Occasionally when navigator is spawned in gazebo it seems to be stuck not fully spawned, with the props floating in the center of the boat and none of the plugins loaded properly. Deleting NaviGator and spawning it again seems to work, so doing this automatically could be a hackey solution. This seems to be related to system resource exhaustion, perhaps when gazebo process is starved or out of memory. Could also be an issue in gazebo_ros