Add "enable" field to mqtt_user/mqtt_acl tables

[Xyon75]

The idea is to "disable" temporary an account (i already used this with the emq postgres auth plugin).

The selector should be something like this

auth.mongo.authquery.selector = username=%u && enable=true
auth.mongo.aclquery.selector = username=%u && enable=true

and the docs something like this:

{username: "suspended_user", password: "password hash", is_superuser: false, enable: false}
{username: "suspended_acl", pubsub: ["mytopic/#"], enable: false}