DefaultKeystoreService cache doesn't add entries to cache

https://github.com/apache/knox/blob/master/gateway-server/src/main/java/org/apache/knox/gateway/services/security/impl/DefaultKeystoreService.java#L457

When there is no clusterName entry in the cache, a new clusterCache map is created. This map is updated but never put back on the cache. This means that the cache is never being updated properly.