Add a cloud build config to upload WAR files to GCS

The private repo structure is re-done to mirror that of the public repo to facilitate easy merging.

Also removed steps to tag the private repos. This will likely cause a race condition if both the nomulus and the proxy cloud build are triggered by the same tag. They will both try to tag the private repo with the same tag.

The tagging of the private repo should happen simultaneously with the public repo tagging, and in an out-of-band process as far as the build process is concerned. The build process should not have side effect on its source.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=225544063

gradle/build.gradle

@@ -84,7 +84,7 @@ subprojects {
     }
 
     rootProject.deploy.dependsOn appengineDeploy
-    rootProject.stage.dependsOn appengineStage
+    rootProject.stage.dependsOn explodeWar
 
     // Return early, do not apply the settings below.
     return