Always include the installer answer file in backups

[ekohl]

If the user has changed the answer file location in their scenario to something outside of /etc/foreman-installer then it isn't backed up.

This changes the installer feature's config_files to include it if needed. It also stops globbing the files and simply trust that you can list a directory, which we already do for the cached data.

I'm unsure about the implications for restores.

Note that this also prepares for the case where we will move the answer files to /var/lib/foreman-installer (provided we keep the scenario files in /etc).