Use nss-altfiles for system users

[iBelieve]

See https://developer.gnome.org/ostree/2014.4/lib-passwd.html. This will allow us to have system users in a separate file from locally created users, allowing us to add and remove them as necessary.

--- Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/20779030-use-nss-altfiles-for-system-users?utm_campaign=plugin&utm_content=tracker%2F15707672&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F15707672&utm_medium=issues&utm_source=github).