Add access to prometheus page via main url

Small change for us mainly. This allows us to access the Prometheus page directly via the environment's hostname instead of having to find the IP. For example with these changes you can now access Prometheus via the hostname and it's port - http://10k.testbed.gitlab.net:9090/.

We can't easily host it on a suburl via load balancing as Prometheus's page is configure to just listen on a port instead of "/prometheus" so we just open the same port on HAProxy and forward from that.