Change policy on fetching configuration variables

[adg]

The gddo-server can fetch configuration variables (github auth tokens, google analytics tracking codes, HTTP user agent string) from either Google Compute Engine's Metadata server or using local environment variables.

Presently, it uses the metadata service if available, and falls back on environment variables otherwise.

@bradfitz points out in a recent code review:

these sorts of checks are annoying when your dev machine is running on GCE itself. Please check the os.Getenv first and only if it's empty, then fall back to checking GCE + the metadata.

So perhaps we should change the policy throughout the codebase to prefer the environment variables and use GCE as a backup.