Create library functions for deriving test environment info

[alanking]

Some things in the test environment are being hard-coded and are quite unlikely to change, but it would be better if we derived some of this information from elsewhere, like, for example, environment variables. Some of the information could include...

• iRODS client environment (host, port, zone, username, password)
• Redis connectivity (host, port, db)
• Celery-related environment variables (e.g. CELERY_BROKER_URL)