Allow usage of enterprise principals
    
    Enterprise principals are currently most useful for the AD provider and
    hence enabled here by default while for the other Kerberos based
    authentication providers they are disabled by default.
    
    If additional UPN suffixes are configured for the AD domain the user
    principal stored in the AD LDAP server might not contain the real
    Kerberos realm of the AD domain but one of the additional suffixes which
    might be completely randomly chooses, e.g. are not related to any
    existing DNS domain. This make it hard for a client to figure out the
    right KDC to send requests to.
    
    To get around this enterprise principals (see
    http://tools.ietf.org/html/rfc6806 for details) were introduced.
    Basically a default realm is added to the principal so that the Kerberos
    client libraries at least know where to send the request to. It is not
    in the responsibility of the KDC to either handle the request itself,
    return a client referral if he thinks a different KDC can handle the
    request or return and error. This feature is also use to allow
    authentication in AD environments with cross forest trusts.
    
    Fixes https://fedorahosted.org/sssd/ticket/1842