Force refreshing Kea configuration from the UI

Stork server does not update Kea configuration in its database when the Kea configuration hasn't changed. This is fine in most cases. However, in cases like #1198 (closed) it would be useful to have an option to invalidate the config expecting the puller to refresh it. Without it, the Stork server will not trigger certain actions (like updating loggers in #1198 (closed) case) and the user won't see the updates until they explicitly modify the Kea configuration.