vagrant-dev-env: Factor out dependencies update from edxapp/deploy
vagrant-dev-env: Structure of edxappdev - Dev version of edxapp role
vagrant-dev-env: Vagrant launcher for development environment VM
vagrant-dev-env: Move vagrant inventory to use default group_vars

`group_vars` is, according to the documentation, the best place to place
default values for variables, as it has the lowest level of precedence:

http://www.ansibleworks.com/docs/playbooks2.html#understanding-variable-precedence

This adds the possibility of reserving the `vars_files` attribute from
the playbook, of higher precedence, to custom values for Vagrant.