ini: Add internal validator ini_allowed_options
    
    Ticket:
    https://fedorahosted.org/sssd/ticket/133
    
    This validator allows to specify a per section
    list of known options. Error message is generated
    if unknown option is found.
    
    Signed-off-by: Lukas Slebodnik <lslebodn@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>