Fixed checking for SuSEfirewall2 package depending on the stage/mode

- In inst-sys, the package needs to be selected
- On a running system Yast can install the package if user approves that
- Otherwise it just checks whether the package is installed
- Includes test case
- bnc#887406
- build passes
And I guess that ignoring ```@check_and_install_packages``` is intentional as well, another thing in which I would appreciate a comment.