Add cifs principal to S4U2Proxy targets only when running ipa-adtrust-install
    
    Since CIFS principal is generated by ipa-adtrust-install and is only
    usable after setting CIFS configuration, there is no need to include it
    into default setup.
    
    This should fix upgrades from 2.2 to 3.0 where CIFS principal does not
    exist by default.
    
    https://fedorahosted.org/freeipa/ticket/3041