Properly deprecate `check_dns`

Instead of having to leave `**kwargs` "forever" in `salt.config.minion_config` and `salt.config.apply_minion_config` in order to support API access to those which still passes `check_dns`. Support the argument but show a deprecation warning once. Leave the deprecation warning for 2 major releases. Then, finally, and properly, remove `check_dns` for good.