Disable searching the 'users' data_bag when defining SMB users

We don't always want to grant every user who has access to the machine an SMB user, and feel that there should at least be an option to not use this feature.
The `users` data bag is now an attribute (see 98ccf067ffd67ab3d4d9eea74ca705c0d9d9dabc), so `users` would need to be changed to the attribute.

The attribute in the second part of the conditional would need to match, so change per my suggestion above.

`== true` is unnecessary here.