add some test for reset network from host to prevent crash

before:
```
docker run -itd --net none --name test busybox:latest
pipework eth0 -i eth0 192.168.0.100/24@192.168.0.1
pipework eth0 -i eth0 192.168.0.101/24@192.168.0.1
```
segmentation fault

----

after:
```
docker run -itd --net none --name test busybox:latest
pipework eth0 -i eth0 192.168.0.100/24@192.168.0.1
pipework eth0 -i eth0 192.168.0.101/24@192.168.0.1
```
continue to do more jobs

----
development environment
```
Ubuntu 14.04.1 LTS
Linux 3.13.0-35-generic #62-Ubuntu SMP Fri Aug 15 01:58:42 UTC 2014 x86_64
Docker version 1.2.0, build fa7b24f
```