Generate sitemap files under $docroot/sitemaps/

This is needed to allow this to continue to work with the new router.  The routing currently relies on a fairly [gnarly regex](https://github.gds/gds/puppet/blob/76ef65a134a51ac9329eecd3f40382f79a396f20/modules/varnish/templates/default.vcl.erb#L50), which won't work with the path segment based scheme used by the router.

I've also updated the how the sitemap generator generates the website_base_url to use Plek (and therefore the `GOVUK_WEBSITE_ROOT` ENV variable).  This will have the advantage of actually working on preview.

**Note:** This depends on https://github.gds/gds/puppet/pull/1044.