Returns an Engine instance configured using environment variables.

If the environment variables are set, but incorrect, an authentication
failure will occur when attempting to run jobs on the engine.
Required Environment Variables:
QUANTUM_ENGINE_PROJECT: The name of a google cloud project, with the
quantum engine enabled, that you have access to.
QUANTUM_ENGINE_API_KEY: An API key for the google cloud project named
Raises:EnvironmentError – The environment variables are not set.