feat(etc-hosts) add support for manage_etc_hosts: localhost

Deploying CoreOS with Vagrant results in a host that cannot resolve its own hostname due to the lack of DNS infrastructure.  This PR implements the `manage_etc_hosts` feature from cloud-init to populate `/etc/hosts` with the loopback address.  This allows the host to resolve its own hostname.

Note this feature is based on https://github.com/number5/cloud-init/blob/master/doc/examples/cloud-config.txt#L447:L482