Add region (AZ) labels to VM infrastructure

There are different HAProxy servers serving different availability zones. The K8s clusters are also now divided by zones but our front-end error rates mostly come from HAProxy. Because of this, we aren't able to easily distinguish AZ specific error rates us-east1-{b,c,d} for HAProxy metrics.

We should add a region label (for consistency we overload region for AZ labeling). The relabeling happens at prometheus, what we can probably do is adjust our chef queries for inventory to filter instances by zone and then add the appropriate label. Or, if we are switching to consul based inventory this will probably be easier.