Multilayer images

[prekucki]

Currently, you can provide a single file with an image as golem.srv.comp.task_package property. This then becomes an argument for --task-package.

Often, applications have quite large parts that don't change and small changing pieces which aren't huge. The idea is to be able to specify multiple images just like in Docker, so they can
be combined into one filesystem.

So if you provide

... --task-package img1.gvmi --task-package img2.gvmi --task-package img3.gvmi

this will result in the root filesystem being a combination of all provided images.