In preparation for having multiple top-level kiwi description files,
expose the ability to define which file to read.
Additionally, tests are updated to use this new flag.
The kiwi boxbuild plugin allows to build the image as normal user
inside of a KVM box. The boxes are provided by the plugin and
fetched once or on update. This is useful to decouple the build
from host operating system requirements. The calling user must
have permissions to run KVM instances. Also see:
https://osinside.github.io/kiwi/plugins/self_contained.html
for setting up the sshfs sharing backend
