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 playbooks/roles in 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.