Code review comment for ~jasonzio/cloud-init:mockSLG

I've launched a CentOS VM and confirmed that there were test failures in master, but not with this branch. Thanks!

(I think it's worth noting that if you run tests in a virtualenv or via tox, you shouldn't hit this issue because `import selinux` won't find anything; this is certainly still worth merging to fix the non-virtualenv case!)