Will/vagrant devstack

This is a prototype for `devstack`, a Vagrant instance for local development.  It's basically a clone of shortstack, with the following modifications:

1) Nginx and supervisor are not installed.  It's way easier to develop/debug when using runserver as a foreground process.  This is also the "typical" Django development workflow.

2) Port-forwarding, folder-sharing, and automatic updating of /etc/host for LMS preview.

3) `edxapp` and `forum` users are given login shells that automatically activate the appropriate Python/Ruby environment.  This provides a flexible way to add more repos, even if they have conflicting requirements.

4) Separated a new `browser` role from `jenkins_worker` to install headless browsers for running the test suite.

The instructions in the README depend on `devstack` settings, which are available in this branch of edx-platform: https://github.com/edx/edx-platform/compare/master...will;local-dev-settings

~~This PR includes changes from #409  (Jenkins worker refactor).  Once that gets merged, I will rebase.~~  Update: done.

Suggested reviewer: @feanil