A good way of resetting the simulation is to make use of the /gazebo/reset_world service provided by gazebo_ros_packages.
Also I see you are using the SITL plugin.. maybe you could find some useful code related to the erlecopter in gym-gazebo. Check folders gym_gazebo/envs/erlecopter and gym-gazebo/examples/erlecopter/. Note that this is not under maintenance anymore, but you can just take a look.