Simplify Vagrantfile - read group_vars/development

Previously there was some duplication between configuring the `Vagrantfile` and also `group_vars/development`. Another downside was the Ansible playbook supports multiple sites but the `Vagrantfile` did nothing to easily allow that.

Now the `Vagrantfile` will read `group_vars/development` and automatically set up all the hosts and synced folders. The only addition needed for this is setting `local_path` on each site.

@fullyint love to get your thoughts on this.
(editing lines above to omit any mention of "step 1 above"...)
Tip: Assuming you leave the default `type: 'nfs'` for `config.vm.synced_folder` (in `Vagrantfile`), see the vagrant docs section ["Root Privilege Requirement"](https://docs.vagrantup.com/v2/synced-folders/nfs.html) for how to avoid having to type your password with every `vagrant up`.