ipatests: fix test_crlgen_manage
    
    The goal of the last test in test_crlgen_manage is to ensure that
    ipa-server-install --uninstall can proceed if the server is the last one
    in the topology, even if it is the CRL generation master.
    
    The current code is wrong because it tries to uninstall the master
    (which has already been uninstalled in the prev test), It should rather
    uninstall replicas[0].
    
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>