Prepare for file based configuration

[tsloughter]

There will soon (after open-telemetry/oteps#225 merges) be work on a spec addition for file based configuration of the SDK.

The unique position we are in and after to answer is how this plays with the application environment variables. For OS env vars it is likely the spec will say those are ignored if a config file is supplied.

To be consistent we probably want to do the same for the application environment.