Test-build clear linux bundle as part of the Travis tests

[dlespiau]

We need better integration with clear linux, automatically testing we can create a bundle in Travis seems like a good start, reducing the bad surprises downstream can have. Hopefully it is possible to do it outside of Intel.

For that, I'd isolate the bundle build into its own Dockerfile so we can easily develop and reproduce the bundle build locally. It'd be a shame no to do that considering the project.