tls proxy: immediately close a connection to the backend

Force mod_proxy to immediately close a connection to the backend
after being used, and thus, disable its persistent connection and
pool for that backend.

Let's see if that helps fixing bug #1630664 (the
Connection aborted/ BadStatusLine thing).

We already have an ER query (in queries/1630664.yaml) that should show
whether this is effective.

Change-Id: I03b09f7df5c6e134ec4091a2f8dfe8ef614d1951
diff --git a/lib/tls b/lib/tls
index 57b5e52..d96ca4f 100644
--- a/lib/tls
+++ b/lib/tls
@@ -520,7 +520,7 @@
     SSLCertificateFile $DEVSTACK_CERT
 
     <Location />
-        ProxyPass http://$b_host:$b_port/ retry=5 nocanon
+        ProxyPass http://$b_host:$b_port/ retry=5 disablereuse=on keepalive=off nocanon
         ProxyPassReverse http://$b_host:$b_port/
     </Location>
     ErrorLog $APACHE_LOG_DIR/tls-proxy_error.log