Support for setting extra-vars from hash (ansible.extra_vars)

Added a new attribute ansible.extra_vars, which supports setting Ansible's extra-vars parameter via a Ruby hash.

This allows you to have this in your Vagrantfile:

      ansible.extra_vars = {
        hosts: server1_config.vm.host_name,
        hostname: server1_hostname,
        cookie_domain: cookie_domain,
        deployment_uri: deployment_uri,
        root_suffix: root_suffix,
      }

Which is more maintainable than cramming everything in the ansible.options attribute.