Allow drupal version to be set via an environment variable.

[Jaesin]

If you have a testing matrix that tests against multiple versions of drupal, it would be nice to be able to set the drupal version via an environment variable.

I understand this image already comes with a version of baked in but maybe we can just overwrite it if a new environment variable is detected.

[deviantintegral]

I think you can do this already with workflows in your config.yml. You can define two different sets of jobs, and they can use totally independent Docker containers, with one pointed at 0.4.0 and one at 0.3.5.

Now that Drupal is supporting multiple branches for security releases, I'd be open to updating the setup script and template to automatically test the current and prior releases.