Add apt_conf_snippet and use it where possible

Add apt_conf_snippet and use it where possible

With the new define, it's easy to add an apt.conf snippet in apt.conf.d
It accepts either 'sources' to get a static file or 'content' to define
content inline or with the help of a template.

Put it to use where we create files in apt.conf.d

Finally, fix the dependancy to the apt_config file (however, I don't see
the need for this dependancy)

Signed-off-by: Gabriel Filion <lelutin@gmail.com>