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.

Partial-Bug: #1263310
Merge pull request #58 from CiscoSystems/bug_1263310

Use template instead of static file for preseed