Handle SIGTERM to enable graceful script shutdown when a container is stopped. This currently only matters for the InfluxDB scripts, and only when they run in a loop, since if the script is hard-terminated, it won't flush out any queued data points to the InfluxDB server. This also required changing the entrypoint script to exec python instead of running it as a child process of the shell running entrypoint.sh, since Docker will only deliver SIGTERM to the parent process it started directly.
Also, add -t 30 to the default Docker command to match the script default behavior prior to the changes in 46f65a6214
