Puppet to bring up Vagrant development environment with a UI front-end.

This is coming from a branch with cherry picked commits from my master.

This should enable a UI desktop and all the relevant build dependencies for SITL using Puppet manifests. Also updated the README.md to match the new Vagrant provisioning.

The linux shell script (prereqs.sh) for Vagrants is no longer needed;  desktop linux (e.g. non-VM) will still need to use the pre-reqs script.  Probably should encourage as many as possible to use Vagrants so development environments are standardized via the Puppet manifests.