CDB-1761 Invalidate using Varnish special HTTP interface.

This performs invalidation using a HTTP interface set on Varnish
default.vcl as follows:

if (req.request == "PURGE") {
  # If the source IP is not in the purge acl, dont't allow it
  if (!client.ip ~ purge) {
    error 405 "Not allowed";
  } else {
    # Bath type invalidation (with regular expression). Similar to the
    # admin interface
    if (req.url ~ "^/batch") {
      purge(obj.http.X-Cache-Channel ~ req.http.Invalidation-Match);
      error 200 "Ban added";
    }
    # It only invalidates the URL that is hitting varnish
    else {
      return (lookup);
    }
  }
}