networker skeleton

worker: networker skeleton

This RP is a skeleton for the future networker based on StringsWorker

Continue of work from: https://github.com/juju/juju/pull/86
This does not actually check if the interface is configured. We need to know whether the interface is known and is it up or down. Instead, using `netstat -ai` will give you a table, you can parse:

```
Kernel Interface table
Iface   MTU Met   RX-OK RX-ERR RX-DRP RX-OVR    TX-OK TX-ERR TX-DRP TX-OVR Flg
eth0       1500 0         0      0      0 0             0      0      0      0 BMU
lo        65536 0    123208      0      0 0        123208      0      0      0 LRU
lxcbr0     1500 0         0      0      0 0           621      0      0      0 BMRU
sit0       1480 0         0      0      0 0             0      0      0      0 O
virbr0     1500 0         0      0      0 0             0      0      0      0 BMU
virbr1     1500 0         0      0      0 0             0      0      0      0 BMU
virbr2     1500 0         0      0      0 0             0      0      0      0 BMU
virbr0-nic  1500 0         0      0      0 0             0      0      0      0 BM
virbr1-nic  1500 0         0      0      0 0             0      0      0      0 BM
virbr2-nic  1500 0         0      0      0 0             0      0      0      0 BM
wlan0      1500 0   5389932      0      0 0       1701787      0      0      0 BMRU
```

Parsing the last column can tell you if the interface is up ("U"), therefore has an address and running ("R"), so it's actually usable. So if you make a helper to run that command, parse the output, and return 3 set.Strings (up, running, others), you can use it to easily check all cases.

There's also the `ifquery` command that parses /etc/network/interfaces and can list them, but it didn't look that useful, and does not tell you whether the interfaces actually work or not.