Missing Dockerfile should not make tests fail

[hhorak]

Sometimes it happens that we have only RHEL packages available and CentOS are not yet build, but we want to submit the PR with new version. Or we might only have Fedora variant available, but no CentOS or RHEL Dockerfiles. So, the testing (building) script should work the way that the testing would not be run (and test whould not fail) in cases the particular Dockerfile variant is missing (e.g. only Dockerfile.rhel7 is available and not Dockerfile for CentOS).