Fix handling of unknown build OSes to not produce errors

We should anticipate end users utilizing any variety of base operating systems that we may not have coded in as BuildOS objects to bakery. Currently, attempting to load a project with an arbitrary OS will result in an error because the build_os.image_tag attribute will be None in ImageVariant.load. We should fix this behavior so users are warned on utilizing an arbitrary OS, but any operations performed with an arbitrary OS still succeed.

How to Reproduce

1. Create a temp directory to test in and use it as working directory.
2. Run bakery create project.
3. Run bakery create image test.
4. Change extension on Containerfile.jinja2 to Containerfile.randomos.jinja2. (See #108)
5. Run bakery create version test 1.0.0. This will propagate an error during tag rendering because attributes for the BuildOS object are not set.

Start at ImageVariant.load and the tag-rendering path used by bakery create version test 1.0.0; compare how known BuildOS objects are handled. Done means an unrecognized Containerfile..jinja2 produces a warning instead of an error and the create-version operation succeeds.