Tern is unable to find host accessible from SSH host

Hi

Hope all is well!

I'm running the following command:
```
tern migrate --conn-string ${CONN_STR} --password ${PASSWORD} --migrations ./db/migrations --ssh-host ${BASTION_PUBLIC_IP} --ssh-user ec2-user
```

and am getting the following:
```
Unable to connect to PostgreSQL:
failed to connect to `host=... user=... database=...`: hostname resolving error (lookup : no such host)
```

I am able to SSH into the host and then perform the DNS lookup successfully.

I suspect tern is performing the DNS lookup locally (where the host is inaccessible) as opposed to on the ssh machine (where the host is accessible).

Start with the `tern migrate` command and its `--ssh-host`/`--ssh-user` options, then reproduce the command with the supplied connection string and bastion setup. Trace where the PostgreSQL hostname is resolved and verify completion by making the database host resolvable through the SSH host rather than the local environment.

Written by the indexing model from the issue text.