Using namespace fds instead of paths and nspids

This change removing hardcode usage of PID namespaces and allows to pass namespaces fd to libcontainer instead.
There will be change in docker with persistent network namespaces for each container.
Lets change this this to a map[string]string or maybe a typed NsPath struct.

The map would look like:
```json
"net": "/proc/38297/ns/net",
"pid":/proc/38297/ns/pid",
...
```

What do you all think?