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