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.
maybe I'm nitpicking here but wouldn't it be more clear to just call it `wordpress_sites` instead of `wp_sites`? Just keep the same name as what is in the config file?