Install refactoring

Ah ok. And it doesn't work with the packages shipped with CentOS?
I think, if possible it is a good thing to use packages, as they are kept up to date automatically using yum/apt, and they are consistent concerning the versions. Otherwise it might break if a maintainer decides to release a new version with a changed behaviour.

It is also good to have to option to install it using `python_pip` in case it is not the case. Maybe we should add an attribute like `node['duplicity_ng']['use_pip']` that defaults to true on centos and false on recent Ubuntus?