Collapse --external-ca-profile tests into single class
    
    To avoid having to spawn new CI hosts for each kind of
    --external-ca-profile argument we are testing, collapse the three
    separate test classes into one.  Uninstall the half-installed IPA
    after each section of tests.
    
    This change is in response to review comment
    https://github.com/freeipa/freeipa/pull/2852#pullrequestreview-220442170.
    
    Part of: https://pagure.io/freeipa/issue/7548
    
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>