Add proxy-initial-not-pooled to proxy

As described in the inline comment, we are seeing infrequent 502
errors in the logs that do seem to match what is described as a
race-condition in the Apache documentation.

As suggested there, enable proxy-initial-not-pooled in an attempt to
avoid these.

It says I"t has to be kept in mind that setting this variable
downgrades performance, especially with HTTP/1.0 clients"
... downgraded performance would be preferable to 502 errors leading
to long-running CI jobs failing randomly; so seems like a trade-off
worth trying.

Change-Id: I94a73e0fc07ca5b8903f4e74ad7c224509cea323

modules/openstack_project/templates/mirror.vhost.erb

@@ -122,6 +122,21 @@ NameVirtualHost <%= @vhost_name %>:8081
     # 5GiB
     CacheMaxFileSize 5368709120
 
+    # Added Aug 2017 in an attempt to avoid occasional 502 errors (around
+    # 0.05% of requests) of the type:
+    #
+    #  End of file found: ... AH01102: error reading status line from remote server ...
+    #
+    # Per [1]:
+    #
+    #  This avoids the "proxy: error reading status line from remote
+    #  server" error message caused by the race condition that the backend
+    #  server closed the pooled connection after the connection check by the
+    #  proxy and before data sent by the proxy reached the backend.
+    #
+    # [1] https://httpd.apache.org/docs/2.4/mod/mod_proxy_http.html
+    SetEnv proxy-initial-not-pooled 1
+
     # Per site caching reverse proxy rules
     # Only cache specific backends, rely on afs cache otherwise.