added subcommand networks to list networks

Example output of this subcommand:

```
$ vagrant rackspace networks
==> default: Network Name                         Network CIDR
==> default: private
==> default: public
==> default: vagrant001                           192.168.3.0/24
==> default: vagrant002                           192.168.4.0/24
==> default: vagrant003                           192.168.5.0/24
```