make users data bag optional

This has been bugging me for some time (see comments in #37 and #16 ): The cookbook raises an error if there is no data bag `users` on the server - which is a perfectly valid scenario: I only want to **harden** my servers, not install users.

This pr fixes that. A non-existing users data bag is treated like an empty one.

As a side effect, this makes a lot of the server tests more concise, since we can avoid stubbing the unused bag.