Allow server env var prefix to be overridden

[daleal]

As suggested here, use a POLICYBOT_ENV_PREFIX environment variable to override the prefix for server configuration values. This makes it possible to run the Docker container on Heroku, where it is expected that the container will listen on the port specified in the PORT environment variable. This PR is basically a clone of the same PR for the Bulldozer repo.

[palantirtech]

Thanks for your interest in palantir/policy-bot, @daleal! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[bluekeyes]

Thanks for adding this!

[daleal]

No problem! Just to understand the timeline, when do you think this will make it to your standard Docker image?

[bluekeyes]

You can try this out using the snapshot tag (palantirtechnologies/policy-bot:snapshot) right now. We'll hopefully tag a proper release within the next week, once we merge some other pending changes.

[daleal]

Nice, thanks!