This reverts commit 247c2bc9.

I'm sorry, I didn't double check what's staged and thus didn't commit fixed
variable names: 'iso_checksum' -> 'iso_checksum_url'

By replacing iso_checksum with iso_checksum_url there is no need to
hardcode hash values into vagrant.json.

Trying to add mirror variable leeds to errors: vars inside variables are not
reusable. See packer issue (#874)[https://github.com/hashicorp/packer/issues/874]

We need a sequence if we want to use the vagrant-cloud post-processor.

I also enabled headless for future headless builds on a buildserver

added polkit to pacstrap for systemctl poweroff, added sudo to poweroff
command, moved link for systemd-resolved to postinstall.sh

added new postinstall.sh, moved the controllers to postinstall.sh and added the script to vagrant.json