WIP: Vagrant stacks use ansible pull to update

First attempt at using the new `edx_ansible` role to simplify the Vagrant release process.

1) Base boxes (in configuration/vagrant/base) are provisioned using playbooks that now include `edx_ansible`.
2) Release boxes (in configuration/vagrant/release) assume that Ansible is installed in the base box, then use a shell script to execute Ansible playbooks on the Vagrant localhost ("ansible-pull" model).

Advantages:
1) Developers using the release stacks don't need to have Ansible installed in their host machines, which greatly simplifies the setup process.
2) Changes to the configuration repo won't break the release stacks.

I still need to do a lot of testing to make sure this works correctly, but I'm interested in feedback on the general approach.