Make CA-less ipa-server-install option --root-ca-file optional.
    
    The CA cert specified by --root-ca-file option must always be the CA cert of
    the CA which issued the server certificates in the PKCS#12 files. As the cert
    is not actually user selectable, use CA cert from the PKCS#12 files by default
    if it is present.
    
    Document --root-ca-file in ipa-server-install man page.
    
    https://fedorahosted.org/freeipa/ticket/4457
    
    Reviewed-By: Petr Viktorin <pviktori@redhat.com>