Extract configuration from Vagrantfile

[shedd]

Right now, Vagrantfiles are a mix of configuration of the guest machine being built + personal local configuration.

As in this commit setting up a synced folder - d215961 - it would be nice to be able to have the specific path extracted out to environment variables or other external configuration.

Then we could have a env_config.yml.example file checked into the repo, making it easy to separate personal configuration from configuration of the guest machine.

[shedd]

I thought the Dotenv gem (https://github.com/bkeepers/dotenv) seemed like a potential option for this, but couldn't get it working within the context of a Vagrantfile. WIP branch here: master...dotenv

[shedd]

Made some improvements by moving "personal" config to the top of Vagrantfile: 1b3175a