Skip provider if it doesn't support single logout
    
    Some providers don't have a singlelogout service, in which case we
    should just move on instead of crashing.
    
    Ticket: #206
    Fixes: #66
    Signed-off-by: Patrick Uiterwijk <puiterwijk@redhat.com>
    Reviewed-by: Rob Crittenden <rcritten@redhat.com>