Ensure duplicate roles are not loaded in the case of a missing meta/main.yml file

Pull request for https://github.com/ansible/ansible/issues/4017

>I was running into issues with a dependency being included multiple times when i thought the default was to onlyinclude once.
>
>I had been trying allow_duplicates: false in the top level role but that had no effect. After stepping through the code in a debugger i noticed that it was looking in the dependency role itself for a meta.yml file which was not present. So now for my situation I can get it working.
>
>However, I wanted to post this issue to see if this is how it is intended.
>
>It seems to work like the following:
>
>Top role depends on Base role.
>
>Base role contains no meta/main.yml => Role can be included many times
>Base role contains empty meta/main.yml => Role only included once.
>Base role contains meta/main.yml with allow_duplicates set => works as expected