REGTESTS: fix the race conditions in normalize_uri.vtc

REGTESTS: fix the race conditions in normalize_uri.vtc

There is no connection reuse to avoid race conditions in HTTP reg-tests. But
time to time, normalize_uri.vtc still report "HTTP header incomplete"
error. It seems to be because HTTP keep-alive is still used at the session
level. Thus when the same server section is used to handle multiple requests
for the same client, via a "-repeat" statement, a new request for this client
may be handled by HAProxy before the server is restarted.

To avoid any trouble, HTTP keep-alive is disabled on the server side by
adding "Connection: close" header in responses. It seems to be ok now. We
let the CI decide.