Fix session logout
    
    There were 2 issues with session logouts, one is that the logout_cookie
    was checked and acted on in the wrong place, the other is that the wrong
    value was set in the IPASESSION header.
    
    Fixes https://fedorahosted.org/freeipa/ticket/6685
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>