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>
Fix README.
Merge remote-tracking branch 'zyga/puppet'

This provisions the VMs with puppet, allowing us to do nice things
like disable the screensaver, and more easily automate the
dist-upgrade to ensure that freshly-spun VMs are always up-to-date.