Add Workstation dvd-ostree flavor and tests
    
    Summary:
    As we're getting the Workstation dvd-ostree (OStree installer
    image) built for Rawhide now, let's try testing it.
    
    Test Plan:
    Run the tests on a Rawhide compose that works and
    has the image (e.g. 20170615.n.0). Check that new tests work
    as expected and old tests are not adversely affected. A
    corresponding diff for fedora_openqa will be coming to take
    care of scheduling. Note that the tests will often soft fail
    for now; this is intentional due to RHBZ#1193590, the bash
    prompt for root is incorrect on ostree installs, so I have
    added a needle that matches the incorrect prompt but which is
    flagged as a workaround needle (so causing the test result to
    be a soft fail).
    
    Reviewers: jsedlak, jskladan
    
    Reviewed By: jsedlak
    
    Subscribers: tflink
    
    Differential Revision: https://phab.qa.fedoraproject.org/D1211