Split external_ca PR-CI into two jobs
    
    The external_ca job takes about 38 minutes of testing. Split the tests
    into TestExternalCA (~17 minutes) and TestSelfExternalSelf +
    TestExternalCAInstall (~20 minutes).
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>