Add note: Ubuntu 12.04 cannot preseed over https

Ubuntu 12.04 LTS cannot load a preseed.cfg file over HTTPS. This patch
adds a warning for enterprising users who want to host their own preseed
files.

Partial-Bug: 1312764
Implements: blueprint openstack-training-labs
Change-Id: I08b74bc0a3edd2b0752c9257e13647f216919007
Merge "Add note: Ubuntu 12.04 cannot preseed over https"