Avoid overwriting existing config from env

[yoursunny]

No description provided.

[TomGlass]

Hey can you hop into IRC irc.hackint.org #archiveteam-dev makes life easier ;)

[yoursunny]

Currently, start.py would always generate a config file from the environment variables.
If the user did not specify environment variables when launching the Docker container, but instead entered their downloader name and project selection via web UI, these changes would be lost if the container is restarted.

This commit modifies start.py so that if an existing config file is found, it would be used instead of the environment variables.
This was the intended behavior specified in README, and my commit restores this behavior.