fix conntrack collector: change to proper path and files

Tested on my Ubuntu 

````
hvn@hvnlxc: ~ () $ uname -a && lsb_release -a
Linux hvnlxc 3.8.0-26-generic #38~precise2-Ubuntu SMP Thu Jun 20 18:29:36 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 12.04.4 LTS
Release:        12.04
Codename:       precise

hvn@hvnlxc: ~ () $ cat /proc/sys/net/netfilter/nf_conntrack_count 
8
hvn@hvnlxc: ~ () $ cat /proc/sys/net/netfilter/nf_conntrack_max
65536
hvn@hvnlxc: ~ () $ cat /proc/sys/net/ipv
ipv4/ ipv6/ 
hvn@hvnlxc: ~ () $ cat /proc/sys/net/ipv4/netfilter/ip_conntrack_max
cat: /proc/sys/net/ipv4/netfilter/ip_conntrack_max: No such file or directory

````

This regression introduced in 484f515bf0dfdb8a5bf13dee916aaf7b70e69aea
I'm not sure author who changed this collector recently tested it on which linux version, so we maybe need discuss on this change. My fix only tested on Ubuntu 12.04.4 kernel 3.8

I know this is a configurable value, but the default config broke my current work well server, so this should be considered as a regression.