Enable rhel/fedora tags until relevancy is ready
    
    Using tags could be an easy temporary workaround for filtering
    rhel-only or fedora-only tests until the test case relevancy is
    implemented. Renamed the attribute to 'tag' according to the spec:
    
        https://tmt.readthedocs.io/en/latest/spec/tests.html#tag
    
    Should work nice both with STI (referencing tests from tests.yml)
    and tmt (for example tmt test ls --filter tag:rhel).