Make policy work with 3.4.x

fixes #283
fix params/repos.json attributes
fix dependency in Repository::apt::Maintain

'Linux' is not the class the cfengine sets for Linux machines, it's 'linux.'
Generate sources file from scratch overwiting mods

fixes #282

The header of the file warns that this happens, this commit justs
increases the warning's truthiness.
fix the wipe in Repository::apt::Maintain

fixes #284
Merge pull request #286 from ph1l/feature-fix_apt_repo

aptrepo: fix bugs in Repository::apt::Maintain