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.
Merge pull request #101 from roots/vagrantfile-updates

Simplify Vagrantfile - read group_vars/development