bro-sandbox: bro-sandbox/ add Vagrantfile and provision script

Create a VM for Bro training.

These files create a VM that uses Docker to provide isolated
user environments as containers for each SSH login for the demo
user. Bro is installed in each of this environments and the container
is removed upon session termination.

Test after provision:
$ ssh -p 2222 demo@127.0.0.1
Merge branch 'master' of https://github.com/jonschipp/vagrant