set datastore project id for needed datastore migration

Author: chingor13

.kokoro/presubmit/common.cfg

@@ -24,6 +24,12 @@ env_vars: {
   value: "test"
 }
 
+# Well known environment variable to datastore tests
+env_vars: {
+  key: "DATASTORE_PROJECT_ID"
+  value: "cloud-java-ci-test"
+}
+
 before_action {
   fetch_keystore {
     keystore_resource {