server-install: No double Kerberos install
    
    When we're installing server with an external CA, the installation
    would have failed in the second step where it's passed the required
    CA cert file because it would have tried to perform the Kerberos
    installation for the second time.
    
    https://pagure.io/freeipa/issue/6757
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>