Set ulimit-n to correct value for 4096 maxconn.

[Haproxy documentation](http://www.haproxy.org/download/1.3/doc/haproxy-en.txt) suggests that you should set `ulimit-n` using global keyword to (2 * maxconn + nbproxies + nbservers + 1).

Otherwise when I run haproxy in docker with the generated config I get

```
[WARNING] 209/130738 (19) : [haproxy.main()] Cannot raise FD limit to 8203.
[WARNING] 209/130738 (19) : [haproxy.main()] FD limit (1024) too low for maxconn=4096/maxsock=8203. Please raise 'ulimit-n' to 8203 or more to avoid any trouble.
```