Register app dns vhost for secondary haproxy gears

Modify the vhost frontend plugin to create a 2nd base vhost conf file
for the app dns so secondary haproxy gears can handle requests for
$OPENSHIFT_APP_DNS

Bug 1161072
Merge pull request #5954 from ncdc/bug/1161072-vhost-multi-ha-app-dns

Merged by openshift-bot