Add --json-attributes option and fix tests with mixlib-config 2

This can be used to set node attributes.

Supported natively by `Chef::Knife::Bootstrap`:

```
    -j JSON_ATTRIBS,                 A JSON string to be added to the first run of chef-client
        --json-attributes
```

Usage example:

```console
$ digital_ocean droplet create \
  --server-name proxy \
  --json-attributes '{ "apache": { "listen_ports": 8080 } }' \
  --bootstrap \
  [...]
```

**Extra:**  	Tests fixed to work with **mixlib-config** `2.0.0`.