Fixed checking for SuSEfirewall2 package

- 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
Added new test case
Published a variable needed for test case
Changes and version
Added explanatory comment
Extended test to reset a cached value and test true/false

- As wanted by Ancor
Merge pull request #283 from kobliha/checking-for-SuSEfirewall2-package

Fixed checking for SuSEfirewall2 package depending on the stage/mode