Smarter SuSEFirewall proposal during installation

* Don't propose to enable the firewall if SuSEFirewall is not selected for installation (bnc#881250)
* Avoid wacky proposals when possible (bnc#886554)
But the rest of the public methods of this class are all CamelCase. I used snake_case for the private one I defined, but mixing both styles in the interface of the class looks weird.