LDAP: Add Windows Server 2012 R2 functional level
    
    https://fedorahosted.org/sssd/ticket/2418
    
    According to http://msdn.microsoft.com/en-us/library/cc223272.aspx a
    Windows Server 2012 R2 has a functional level set to '6'. We need to
    support that value in order for tokenGroups to be functional.
    
    For more information on the functional levels, please refer to:
        http://technet.microsoft.com/en-us/library/understanding-active-directory-functional-levels%28v=ws.10%29.aspx
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>