Add option --non_live_versions to set_num_instances command

This commit introduced a new flag to enable SetNumInstancesCommand to
be able to set the number of instances for all non-live versions for
a given service or for all deployed services.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=222826003

java/google/registry/tools/AppEngineConnection.java

@@ -66,10 +66,21 @@ class AppEngineConnection {
   }
 
   enum Service {
-    DEFAULT,
-    TOOLS,
-    BACKEND,
-    PUBAPI
+    DEFAULT("default"),
+    TOOLS("tools"),
+    BACKEND("backend"),
+    PUBAPI("pubapi");
+
+    private final String serviceId;
+
+    Service(String serviceId) {
+      this.serviceId = serviceId;
+    }
+
+    /** Returns the actual service id in App Engine. */
+    String getServiceId() {
+      return serviceId;
+    }
   }
 
   /** Returns a copy of this connection that talks to a different service. */