Upgrade with os-core-common lib 0.3.18

Because of the breaking change in the upgrade of os-core-common v0.3.18, the method getAsJsonObject() in HttpResponse no longer exists. This change causes the storage-gcp service to fail on its DatastoreAccesTokenCreator and GcsCredential class. Fixed by replacing this method with newly added HttpResponseBodyMapper bodyMapper. This change shall not break anything that relies on it.