Fix traceback on non-yum systems

3b8b6de added yum.rpmtrans.RPMBaseCallback as a base class for
the _YumLogger class, which causes tracebacks on non-yum systems. This
commit fixes that by only creating this class if the yum python
interface is successfully imported.
Merge pull request #5819 from terminalmage/quickfix

Fix traceback on non-yum systems