Run su commands with login shell

We had an issue with our integration test that ruby wanted to change to
/root which it couldn't access.
The reason for this was a FileUtils.cd block, which tried to change back
to root, because the environment variables were the same as for root.
Running su with the "-l" parameter fixes this issue.
Merge pull request #11 from SUSE/run_su_commands_with_login_shell

Run su commands with login shell