Initial support for a Cloudstack builder

Add support for Apache Cloudstack (https://cloudstack.apache.org) as a
builder. Supports creating new templates of existing templates as well
as booting from ISO to PXE boot a full installation to an empty block
device.

To be able to bootstrap a template from an empty block device we need
to be able to netboot and kickstart a OS installation. To do this we
spin up a HTTP web server to send files to the virtual machine. We
assume the virtual machine will be booting from a iPXE ISO which will
chain boot off the userdata we provide when deploying the virtual
machine.

This also supports passing in templated userdata to be downloaded from
the booted virtual machine.