Defaults to keeping old configuration, avoids hanging

Using pkg.installed with a '.deb' source uses dpkg instead of apt-get. The package managers ask if you want to keep an old configuration file rather than create a new one, if an old one exists. apt-get already chooses to keep the old one as a default, but dpkg does not. I think this is a sensible default and added it to avoid hanging when installing from '.deb' sources.