Use puppet to build desktop images

Some of the changes from the original shell scripts:
* VirtualBox starts in GUI mode
* GuestOS get 64MB of VRAM
* GUestOS uses 1GB of RAM
* GuestOS gets X11 additions for VirtualBox
* LightDM automatically logs in 'vagrant'
* LightDM starts after installation
* Screen saver and screen lock are disabled

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>