google: add DefaultCredentials function

This new function allows reading the project ID from a service account
JSON file without an additional disk read.

Change-Id: I1f03ca3ca39a2ae3bd6524367c17761b0f08de45
Reviewed-on: https://go-review.googlesource.com/32876
Reviewed-by: Jaana Burcu Dogan <jbd@google.com>

google/appengine_hook.go

@@ -10,4 +10,5 @@ import "google.golang.org/appengine"
 
 func init() {
 	appengineTokenFunc = appengine.AccessToken
+	appengineAppIDFunc = appengine.AppID
 }