DNS list enhancements

Updated CLI dns tool to list zones contents and filter by various fields.

```
usage: sl dns list [<zone>] [options]

List zones and optionally, records

Filters:
  --type=TYPE    Record type, such as A or CNAME
  --data=DATA    Record data, such as an IP address
  --record=HOST  Host record, such as www
  --ttl=TTL      TTL value in seconds, such as 86400
```

Includes new DNSManager.get_records()