(MODULES-1200) Fix inconsistent header across files

Currently, the file headers are not consistent. Most of the files do not even have header, due to the usage of the content attribute of the file resource.

This PR fixes that, all files are now ERB templates and a single header is used across all files.
I tried to follow the request in the referenced ticket to use a concat style, e.g. global setting to enable/disable/modify the header.
However, I found no proper way to do this. This module consists of a lot of subclasses, which can be used independently of the main apt class.
Even with a global parameter, I had troubles with parse order. Adding this parameter to every subclass seems not like a clean way either.

My current implementation is a compromise with the *_header.erb* file.

One solution might be Hiera (quick hack: stdietrich/puppetlabs-apt@c9266d26d6ee9a63865480764f628902be500b13)