Add simple Vagrantfile

I'd rather not install a bunch of packages on my machine to test out Flow. This allows Flow to be installed via [Vagrant](https://www.vagrantup.com/) with a simple `vagrant up` so it can be easily run in a virtual machine.

An outstanding issue is that `local_server.sh` doesn't wait for MongoDB to start, so in my case Flask exits immediately after it can't contact the DB. However, this seems unrelated to Vagrant.