Allow keepalives on status port

A few years ago, #3684 changed the status port to use Unix
sockets rather than TCP. As part of that change, `keepalive_timeout 0`
was added.

Sometime later, #4487 changed the status port back to TCP.
However, keepalive was never re-enabled.

In practice we're seeing behavior where nginx stops accepting
GET or POST to the status port during graceful shutdown
since new connections aren't allowed.

rootfs/etc/nginx/template/nginx.tmpl

@@ -654,7 +654,6 @@ http {
         listen 127.0.0.1:{{ .StatusPort }};
         set $proxy_upstream_name "internal";
 
-        keepalive_timeout 0;
         gzip off;
 
         access_log off;