Packages/Pkgng: fix arguments of __init__

This plugin was developed for 1.3.x and during the transition to master
the changed arguments of Collection.__init__ were missed.
Packages: enable Pkgng backend by default
doc: add simple documentation for packages:backends
Merge branch 'fix-pkgng' of https://github.com/AlexanderS/bcfg2
Merge branch 'doc-packages-backends' of https://github.com/AlexanderS/bcfg2