ipa-server-install: fix uninstall
    
    ipa-server-install --uninstall fails to stop tracking the certificates
    because it assigns a tuple to the variable nicknames, then tries to
    call nicknames.append(). This is a regression introduced by 21f4cbf8.
    
    Assignment should be done using nicknames = list(self.tracking_reqs) instead.
    
    https://pagure.io/freeipa/issue/6950
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>