Set the certmonger subject with a string, not an object
    
    ipa-server-certinstall goes through a slightly different code path
    if the replacement certificate is issued by IPA. This was setting
    the subject using cert.subject which is a Name object and not the
    string representation of that object. This was failing in the
    dbus call to certmonger.
    
    https://pagure.io/freeipa/issue/8204
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>