always retry proxy errors

When an apache worker gets a proxy error, it will not retry talking to
the backend server until the retry timeout expires. We bring up the
proxy server *before* the backend server, and poll it. If we are
running a small number of workers, there is a likely chance that we're
going to hit one that errored before the backend was up, thus failing
for now real reason.

Set this to 0 instead to mean always retry failed connections.

Change-Id: I9e584f087bd375f71ddf0c70f83205c425094a17
Ref: https://httpd.apache.org/docs/2.4/mod/mod_proxy.html#proxypass
diff --git a/lib/tls b/lib/tls
index c2c92a1..7a7b104 100644
--- a/lib/tls
+++ b/lib/tls
@@ -512,7 +512,7 @@
     KeepAlive Off
 
     <Location />
-        ProxyPass http://$b_host:$b_port/ retry=5 nocanon
+        ProxyPass http://$b_host:$b_port/ retry=0 nocanon
         ProxyPassReverse http://$b_host:$b_port/
     </Location>
     ErrorLog $APACHE_LOG_DIR/tls-proxy_error.log