x/build: configure Stackdriver alerting for farmer.golang.org/#health

[bradfitz]

When I added the coordinator health checking, I made each health item have its own URL, linked from https://farmer.golang.org/#health ...

• https://farmer.golang.org/status/macs
• https://farmer.golang.org/status/scaleway
• https://farmer.golang.org/status/basepin

... etc

It returns:

• HTTP 500 if there are any errors
• HTTP 200 + "ok" if everything is perfect
• HTTP 200 + warnings on warnings

That should make it easy to plug in to our existing Stackdriver alerting, to send emails when things are unhealthy.

If the HTTP status + body aren't sufficient to make the Stackdriver integration trivial, adjust the coordinator such that it is.

/cc @dmitshur @andybons