implement a user blacklist

There is a request from customer to have a blacklist of users on maxscale side.

The user still have grants on db, it just need to got stop if go by maxscale. As for Markus discussion, at the moment the implementation of the firewall filter, "could use but it would allow users to connect but prevent any queries", so the black list is something that indeed also prevent the connect.

Remapping the grants on maxscale is not the case, it will cost a lot in case, but implementation of black list it should guarantee the transparency of the grants that are loaded from the database.