In certain build projects, multiple Dockerfiles exist (for example,
one per distro) to simplify reading. However, this role is hardwired
to use dockerfiles only named "Dockerfile".
This is a problem, as you can't override the filename neither per
image, or globally.
This should fix the problem, allowing certain images to be build
by providing the dockerfile argument in docker_images, but also
have a globally overridable flag if you are using a different
convention (for example Dockerfile.distro_minordistroversion).
Change-Id: I075c365bc9f4f85f9ada832d22d1f1e213e68e21
0e50a2bb89