This patch adds support for running tests on multiple images. Instead of
parsing just 1 image, it attempts to always parse multiple images from
config files and/or CLI.
To run dox on multiple images from the CLI, you'll have to:
$ dox -i infra/trusty,infra/f20
The tox.ini `docker:images` option is now a comma-separated list of
images, whereas the dox.yml one is an array of images.