ipatests: enhance TestSubCAkeyReplication
    
    enhance the test suite so that it covers:
    - deleting subCAs (disabling them first)
    - checking what happens when creating a dozen+ subCAs at a time
    - adding a subCA that already exists and expect failure
    
    Related: https://pagure.io/freeipa/issue/8488
    Signed-off-by: François Cami <fcami@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Ondrej Mosnacek <omosnace@redhat.com>
    Reviewed-By: Lukas Vrabec <lvrabec@redhat.com>
    Reviewed-By: Zdenek Pytela <zpytela@redhat.com>
    Reviewed-By: Thomas Woerner <twoerner@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Ondrej Mosnacek <omosnace@redhat.com>
    Reviewed-By: Lukas Vrabec <lvrabec@redhat.com>
    Reviewed-By: Zdenek Pytela <zpytela@redhat.com>
    Reviewed-By: Thomas Woerner <twoerner@redhat.com>