Add fmf-tests on testing-farm jobs support
    
    Add a trigger job running on a Zuul executor node.
    It detects whether a .fmf directory exist and if not tell
    Zuul to skip the job called rpm-tmt-test-on-testing-farm.
    
    Add rpm-tmt-test-on-testing-farm with all branches matcher.
    
    Expect 3 roles in fedora-zuul-jobs:
    - testing-farm-secret
    - testing-farm-run-test
    - testing-farm-fetch-artifacts
    
    Depends-on: https://pagure.io/fedora-zuul-jobs/pull-request/74