Fails on compute node at net.netfilter.nf_conntrack_max

[Sam Stoelinga]

Running salt "cmp01*" state.sls opencontrail results in the error seen below:

```
          ID: net.netfilter.nf_conntrack_max
    Function: sysctl.present
      Result: False
     Comment: Failed to set net.netfilter.nf_conntrack_max to 256000: sysctl net.netfilter.nf_conntrack_max does not exist
     Started: 19:46:11.413325
    Duration: 34.383 ms
     Changes:
```

Environment of compute node:
lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 14.04.5 LTS
Release: 14.04
Codename: trusty