Set default realm for enterprise principals
    
    Enterprise principals require that a default realm is available. To
    make SSSD more robust in the case that the default realm option is
    missing in krb5.conf or to allow SSSD to work with multiple unconnected
    realms (e.g. AD domains without trust between them) the default realm
    will be set explicitly.
    
    Fixes https://fedorahosted.org/sssd/ticket/1931