Show short address information in carbon-lookup

The carbon-lookup tool can be more useful in a scripting environment by only returning the IP address or hostname of the matched nodes. This allows scripts to skip a common grep or awk pipe when consuming this information.

```
% carbon-lookup my.metric
127.0.0.2:2004:1
```

and

```
% carbon-lookup -s my.metric
127.0.0.2
```

Think,

```bash
for node in $(carbon-lookup -s "$metric") ; do
    ssh carbon@$node -- do_something_cool
done
```