Make the distrib a parameter (like the backend)

[mickours]

As distrib tamplate files are basically only global variables make a mechanism to set it as parameter (like backend is set) but using global include: special function.

for example:

1. move all distrib recipe global section to a global
2. run the build with parameters:

kameleon build from_image/base.yaml --global distrib:archlinux --global backend:qemu

This feature is not working as is: cli globals have to be applied before the global include function is applied.