Replace config flags with equivalent config file or env vars

We're simplifying the CLI flags on the `bufstream serve` command in a future release, and we'd like to ensure Jepsen continues to work on the newer version. All of the `--config.*` flags plan to be removed.

This will require making the following changes:

### `--config.name`

Set via the top-level `name` attribute in the `bufstream.yaml` config file (passed to the `--config / -c` option):
```yaml
name:
```

This value also is set to the value of the `HOSTNAME` environment variable if not set in the config.

### `--config.connect_public_address.host`

Replace with the environment variable `BUFSTREAM_CONNECT_PUBLIC_HOST` or set via the config file:
```yaml
connect_public_address:
host:
```

### `--config.kafka.public_address.host`

Replace with the environment variable `BUFSTREAM_KAFKA_PUBLIC_HOST` or set via the config file:
```yaml
kafka:
public_address:
host:
```

### `--config.zone`

Set via the config file:
```yaml
zone:
```

If the config file doesn't include a `zone`, it is automatically detected based on k8s node labels or AWS/GCP metadata. Often times users running on those platforms don't need to set this value.