Remove grpcurl and grpcio-tools from container configuration and add yagrc, so that the direct reflection support in the Python scripts can be used. Also, pin all Python packages, including package dependencies, to specific version numbers, since that was already the case by happenstance due to the way Docker caches its build images and an undesirable version of the protobuf package was being cached.
Addresses #23, which was directly about this change.
Expected to also address #22, as a result of pinning the protobuf package version.
Should also prevent a recurrence of #18, since yagrc will automatically get any new dependant protocol files via reflection.
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
