Run integration tests with credentials from environment

Created by: omarkhan

The integration tests are currently only run if a .env.integration file is present. We run integration tests on CircleCI by copying this file over from the stage server before running the tests. We would like to store credentials on CircleCI directly, but for that to work the integration tests must be able to run without a .env.integration file. This pull request updates the Makefile to allow the integration tests to run with credentials from the environment, if the OPENSTACK_USER environment variable is set.