Add command `pecrant destroy`

Example:

```
$ pecrant destroy                                                                                                                                                                                                                                                   2.1.2
    foobar: Are you sure you want to destroy the 'foobar' VM? [y/N] y
==> foobar: Destroying VM and associated drives...
==> foobar: Running cleanup tasks for 'puppet' provisioner...
    default: Are you sure you want to destroy the 'default' VM? [y/N] y
==> default: Destroying VM and associated drives...
==> default: Running cleanup tasks for 'puppet' provisioner..
$
```