manage_repos sets 'ensure' in apt::source

I have extended the behaviour of the *manage_repos* parameter without affecting the backward compatibility.

*manage_repos* now ensures the apt::source is present or absent based on its boolean value. 

This also allows Debian users to install a specific version of rabbitmq-server without adding an apt source. This code is within *repo/apt.pp* and was previously skipped when *manage_repos=false*

I have also updated the relative rspec tests.