Add DNS support to loom_base

Currently, we only support dnsimple, but any DNS provider which can be modified via API can be added. By default, this does nothing, so it's safe to add everywhere. To use this, a user would need to add JSON to his `base` service in Loom such as:

```json
{"loom_dns":{"provider":"dnsimple"}}
```

This would require the use of a `creds` data bag with a `dnsimple` item. These are configurable, and someone can choose to provide an username/password combo, instead.

```json
{"loom_dns":{"provider":"dnsimple","dnsimple":{"use_databag":false}},"dnsimple":{"username":USERNAME,"password":PASSWORD}}
```
Why `node['loom_dns']['dnsimple']` and not just a check directly on `node['dnsimple']`?