Prevent Tests from creating users on Prod Index

The integration tests had previously used the environment variable
DOCKER_INDEX_URL but it was apparently removed several months ago.

Changed Auth.IndexServerAddress() to return the offical index url
only if DOCKER_INDEX_URL is not set in the environment.

Adjusted expected output in integration test.
Created unit test case for setting the environment variable.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)