Allow to customize butane config file

[tobiashuste]

This way we can support other operating systems than Flatcar Linux as well, e.g. Fedora CoreOS.

[christianhueserhzdr]

Thank you very much for making the butane config file customizable. I would do it the other way round and introduce the butane config file name as a variable instead of the template file name. Could this also be a way to go?

[tobiashuste]

Thank you very much for making the butane config file customizable. I would do it the other way round and introduce the butane config file name as a variable instead of the template file name. Could this also be a way to go?

This change was intended to allow for supporting more than only Flatcar Linux as Operating System. We could als support https://fedoraproject.org/coreos/ with this change. Thus, the template was important to be changeable. The server side file does not really matter. The template file variable could also contain directories.
I suggest to give the variable a better name: gitlab_runner_butane_config_template