added subcommand servers to list servers

Example output of this subcommand:

```
$ vagrant rackspace servers
==> devstack: Server Name          State                IPv4 address
==> devstack: devstack             ACTIVE               166.78.18.8
```
Merge branch 'subcommand_list_servers' of github.com:berendt/vagrant-rackspace into berendt-subcommand_list_servers

Conflicts:
	lib/vagrant-rackspace/action.rb
	lib/vagrant-rackspace/command/root.rb
Merge branch 'berendt-subcommand_list_servers'