Make BUILD_IMAGES a setting instead of a constant

[humitos]

At the moment BUILD_IMAGES (https://github.com/rtfd/readthedocs.org/blob/c7e4f538b4364964d1b694820e38de3b973a26a3/readthedocs/doc_builder/constants.py#L31) is a constant and since sometimes we need to use a custom docker image (as pointed in #2692) we need to modify it to set the new supported versions in the image.

In my case, I needed to add this key:

    'readthedocs/build:dev': {
        'python': {'supported_versions': [2, 2.7, 3, 3.5]},
    },

So, we need to check all the places where this constant is used and replace it for the new setting called DOCKER_BUILD_IMAGES plus add a default in the base.py setting file.