Add preseed files for Ubuntu 12.04.4 LTS installer

These preseed files take the Ubuntu 12.04.4 LTS installer through the
first stage of installation.

The three files differ in what is configured when the freshly installed
system reboots:

preseed-ssh.cfg:
  user osbash (password: osbash) can log in with the insecure Vagrant
  key and use password-less sudo for all commands.

preseed-vbadd.cfg
  VirtualBox guest additions are installed from /dev/sr1 (will
  be put there by install script); a script is installed to run
  activate_autostart.sh from an auto-mounted shared folder upon reboot.

preseed-all.cfg:
  Enables both VM access methods described above.

Partial-Bug: 1312764
Implements: blueprint openstack-training-labs
Change-Id: I37752b7311f7e41b1a798a711f2059ab4ac51230