Fill ipakrbprincipalalias on upgrades
    
    From IPA 3.0, services have by default ipakrbprincipal objectclass which
    allows ipakrbprincipalalias attribute used for case-insensitive principal
    searches. However, services created in previous version do not have
    this objectclass (and attribute) and thus case-insensitive searches
    may return inconsistent results.
    
    Fill ipakrbprincipalalias on upgrades for all 2.x services. Also treat
    Treat the ipakrbprincipal as optional to avoid missing services in
    service-find command if the upgrade fails for any reason.
    
    https://fedorahosted.org/freeipa/ticket/3106