Some small fixes

Hi,

here are some small fixes to the apt cookbook.

- Added attributes/default.rb to give cacher_client a default value
- Removed files/default/apt-proxy-v2.conf because it's not needed anymore
- Removed include of cacher-client in cacher-ng, because the user should decide whether the node is a client or not
- Fixed warnings reported by foodcritic
- Changed code to better conform to Cookbook Style Guide

Philipp