Make unmanaged-files integration tests more stable

Create the file /var/lib/zypp/LastDistributionFlavor during image
building because otherwise the unmanaged-file inspector tests could fail
if run alone because this file seems to be created by another zypper
command which is run before during the previous inspection tests.
Merge pull request #411 from SUSE/make_unmanaged_integration_tests_more_stable

Make unmanaged-files integration tests more stable