Switch Arch Linux to use systemd.

This replicates the original work by @aespinosa in #167, but after rebasing and using the repaired systemd unit. I've tested to make sure I didn't break Fedora & CentOS, but I don't have an Arch Linux box to test on, and DigitalOcean seems to have deprecated all theirs. So if @aespinosa or others would like to test this before I merge it, that'd be appreciated.
/etc/conf.d doesn't exist in arch by default anymore .  I get the following 

```
    Chef::Exceptions::EnclosingDirectoryDoesNotExist
    ------------------------------------------------
    Parent directory /etc/conf.d does not exist.
```

I guess that needs to be fixed upstream in value_for_platform_family() or something.