Kinda like Fedora
I admit it, I'm a little jealous of the Fedora feature of being able to install signed packages without a password prompt. I set out to get close on Ubuntu. The way that you edit the PolicyKit practices towards package install is to edit the file /usr/share/polkit-1/actions/. If you look at the action for "Install packages" you can change <allow_active>auth_admin_keep</allow_active> to <allow_active>yes</allow_active>. Then software center works as expected.