cmd/coordinator: fix staging instance

- Make URLs point to correct external IP
- Disable windows-amd64-2008 builder type that doesn't exist in
  the staging farm

Updates golang/go#18817

Change-Id: Id64a63694f90e70c4fd78f9d1433ed5031822111
Reviewed-on: https://go-review.googlesource.com/42850
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/coordinator/coordinator.go

@@ -361,7 +361,6 @@ func stagingClusterBuilders() map[string]dashboard.BuildConfig {
 		"linux-arm-arm5spacemonkey",
 		"linux-amd64",
 		"linux-386-387",
-		"windows-amd64-2008",
 		"windows-386-gce",
 	} {
 		if c, ok := dashboard.Builders[name]; ok {

cmd/coordinator/gce.go

@@ -102,9 +102,11 @@ func initGCE() error {
 		projectZone = path.Base(projectZone)
 		buildEnv.Zone = projectZone
 
-		buildEnv.StaticIP, err = metadata.ExternalIP()
-		if err != nil {
-			return fmt.Errorf("ExternalIP: %v", err)
+		if buildEnv.StaticIP == "" {
+			buildEnv.StaticIP, err = metadata.ExternalIP()
+			if err != nil {
+				return fmt.Errorf("ExternalIP: %v", err)
+			}
 		}
 
 		if !hasComputeScope() {