health: Make cache health check work with mcrouter.

McRouter sends a SERVER_ERROR for keys starting with a double underscore
except for keys in the special "__mcrouter__" namespace. We were ignoring the
cache miss anyway, so lets just check for mcrouter's health everywhere and
we'll still get the desired result.

r2/r2/controllers/health.py

@@ -69,7 +69,9 @@ class HealthController(MinimalController):
                     # libmemcached doesn't support UDP get/fetch operations
                     continue
                 mc = pylibmc.Client([server], behaviors=behaviors)
-                mc.get("__health_check_%s__" % server)
+                # it's ok that not all caches are mcrouter, we'll just ignore
+                # the miss either way
+                mc.get("__mcrouter__.version")
                 results[server] = "OK"
             except pylibmc.Error as e:
                 g.log.warning("Health check for %s FAILED: %s", server, e)