Document environment variables

[mausch]

Just got an "unknown user" error, when I went to google it I landed on https://github.com/Netflix/metaflow/issues/45 I see in a comment there that there's a hierarchy of environment variables to set this: https://github.com/Netflix/metaflow/issues/45#issuecomment-564555935

However If I google "METAFLOW_USER" I get four results: two of them on github issues and two on gitter. If I google "metaflow environment variables" again I don't see any documentation for this.

There should be a page somewhere with reference documentation including this.

[savingoyal]

Good point. We should add that to the docs.

[mausch]

Just found https://github.com/Netflix/metaflow/blob/master/metaflow/metaflow_config.py so adding a few more comments in there and linking it from the documentation would be great too.

[ayorgo]

I'd also add METAFLOW_PRODUCTION_TOKEN and METAFLOW_RUNTIME_NAME to the list.