Update README to mention govuk-provisioning
Update README to mention govuk-provisioning

We're no longer using vcloud-templates for node definitions as of
alphagov/vagrant-govuk@530ff06da4142f7739dce2d64fa6cbd74eaac7bd.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet