Temporary fix for missing /etc/environment

A fix [0] was merged to coreos-overlay last night, but it is not yet in any released images. Without it, the coreos-cloudinit-vagrant.service unit will fail to run, leaving VMs completely unprovisioned. This is a temporary measure to allow coreos-cloudinit services to run like they used to.

Fix #56

[0] https://github.com/coreos/coreos-overlay/commit/68f8f060f41a70a6120ac33e7866dbd2367dfec5