TLS proxy: disable HTTP KeepAlive

There's a race condition when a client makes a request "at the same
time" the HTTP connection is being closed by Apache because the
`KeepAliveTimeout` is expired.

This is explained in detail and can be reproduce using
https://github.com/mikem23/keepalive-race or
https://github.com/JordanP/openstack-snippets/blob/master/keepalive-race/keep-alive-race.py

Just disable KeepAlive to fix the
('Connection aborted.', BadStatusLine("''",)) error we are seeing.

Change-Id: I46e9f70ee740ec7996c98d386d5289c1491e9436
(cherry picked from commit 4370925181)

lib/tls

@@ -519,6 +519,10 @@ $listen_string
     SSLEngine On
     SSLCertificateFile $DEVSTACK_CERT
 
+    # Disable KeepAlive to fix bug #1630664 a.k.a the
+    # ('Connection aborted.', BadStatusLine("''",)) error
+    KeepAlive Off
+
     <Location />
         ProxyPass http://$b_host:$b_port/ retry=5 nocanon
         ProxyPassReverse http://$b_host:$b_port/