Add $upstream_hostname to proxy_vhost define

Nginx uses the name of the upstream as the proxy hostname unless you
explicitly tell it otherwise. We do this by forwarding the incoming
hostname for all vhosts except graphite. This parameter allows us to
explicitly set the upstream hostname for the graphite vhost.
Set graphite upstream_hostname to graphite

This is the hostname expected by the vhost on monitoring-1. Before this
commit the proxied hostname was graphite-vhost-upstream.
Merge pull request #417 from alphagov/fix-graphite

Fix graphite