mw/proxy: simplify google code (#1019)

* mw/proxy: simplify google code

Minimize bootstrap code a bit, and block on the first resolve of the
google https endpont. Add more logging and include actual error in the
returned errors.

Also re-resolve every 120 seconds, instead of 300 (might eventually make
this an option).

* fix test

middleware/proxy/setup.go

@@ -30,7 +30,8 @@ func setup(c *caddy.Controller) error {
 
 	c.OnStartup(OnStartupMetrics)
 
-	for _, u := range upstreams {
+	for i := range upstreams {
+		u := upstreams[i]
 		c.OnStartup(func() error {
 			return u.Exchanger().OnStartup(P)
 		})