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.
Move to templates/default/arch/conf.d/chef-client.erb so the the template can be picked up by the recipe