Makes use of new gen-source-tar script to provide a source based
keystone image.

This patch introduces a new concept of a .buildinfo file, which provides
variables specific to the image being built which cannot be defaulted in
a generic way, at least until the build script is overhauled. This file
is sourced before the .buildconf which still leaves everything
customisable by the user.

Co-Authored-By: Paul Bourke <paul.bourke@oracle.com>
Change-Id: I0ec6b9cfe6d4173f3192e4ae5833fb0f6d333f1c
Implements: blueprint install-from-source