Initial support for provisioning SLE12 nodes

> Hm. Doesn't suse_get_repos_from_attributes (and thus the attribute map itself) also need to take version into account?

Currently not I think, as the ```repo_names``` (see below) are unique across all versions. This might change when we implement https://trello.com/c/3an3cmlY (separate subdirectories in ```/srv/tftpboot/repos/``` per platform and version)