Fixes label values for Prometheus status queries (#254)
Fixes label values for Prometheus status queries (#254)

A while back we changed the value of the `cluster` label from
"platform-cluster" to just "platform", but the mlab-ns queries were not
updated to reflect this. Fortunately, this only affected metrics that
will very rarely affect a site's status.
m-lab/mlab-ns,m-lab/mlab-ns,m-lab/mlab-ns,m-lab/mlab-ns