Feature Request: Add custom known_hosts file to doctl compute ssh

As the title already says, I'd like to specify a custom `known_hosts` file as I would do with
```bash
> ssh -o UserKnownHostsFile=/tmp/known_hosts user@123.123.123.123
```

Currently the `doctl compute ssh` command does not support this.

The main reason for this is an automated script that gets the IPs of all current droplets and calls `ssh-keyscan` on each one to avoid this interaction.

```bash
The authenticity of host '123.123.123.123' can't be established.
Are you sure you want to continue connecting (yes/no)?
```

I would like to use this feature like this in a loop. The interaction of adding the host each time does not really work in that case.

```bash
for my-droplet-name in droplet-names; do
doctl compute ssh --ssh-known-hosts /tmp/known_hosts \
--ssh-command "do stuff here" \
$my-droplet-name
done
```

In this case, `/tmp/known_hosts` was filled by another script beforehand.

I'd be willing to create a PR for this, if the feature is wanted. After a quick code check, it looks like it is more or less just passing arguments through the call stack