Download latest akanda ISO from the private bucket

Move the akanda ISO download later in the installation
sequence and get the file from the private bucket by
using a credentials file not managed in git.

Change-Id: I7f7b210a6944efaad2c5296f10c41d2d16de39f7
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Merge pull request #2 from dhellmann/move-iso-to-private-bucket

Download latest akanda ISO from the private bucket