Attempt to not need erlang_cook fact

The rabbitmq_erlang_cookie fact is problematic because it represents a security object. Putting it in facts leads to those facts being in puppetdb, which leads those facts to be rendered by something like puppetboard.

The module's use of this fact is actually quite limited, and I've used cleverness to add the checking logic to the exec resource. This makes the fact unnececssary. I've used the unless parameter specifically because it prevents the expanded $rabbitmq_cookie from ending up in the report.
I don't think we can remove the fact right away without a major version bump. I wonder if there is a good way to add a deprecation warning somewhere...