Terminate the ephemeral docker daemon with a SIGTERM

Due to a bug with using DockerInDocker (https://github.com/jpetazzo/dind/issues/19) it is important for us to terminate the ephemeral docker daemon running inside of our container with a SIGTERM. This should hopefully clean up lookback devices to avoid that issue.

This should be as simple as reliably calling `terminate()` on [proc](https://github.com/duedil-ltd/portainer/blob/master/portainer/app/executor.py#L72) when the executor chooses to die.