Use template instead of static file for preseed

Previously for Havana we had a hardcoded, static preseed file.
This commit moves to a template so we can use variable expansion,
and reconciles a few changes that were in the old static file
with the new template.

Note: I'm leaving the old static file in place temporarily for reference.  There may also need to be a few changes to user data for which I'll send along some changes in a bit.

Partial-Bug: #1263310