How to start kafka broker with sasl_plaintext auth

Kafka SASL_PLAINTEXT auth configure

Support SASL_PLAIN auth for kafka broker

Kafka broker can be configure like
`/usr/local/etc/kafka/server.properties`:

```
listeners=SASL_PLAINTEXT://localhost:9093
advertised.listeners=SASL_PLAINTEXT://localhost:9093
security.inter.broker.protocol=SASL_PLAINTEXT
sasl.mechanism.inter.broker.protocol=PLAIN
sasl.enabled.mechanisms=PLAIN
```
JAAS file `kafka_server_jaas.conf`:

```
KafkaServer {
org.apache.kafka.common.security.plain.PlainLoginModule required
username="admin"
password="admin-secret"
user_admin="admin-secret"
user_alice="alice-secret";
};
```
Export Kafka Options before start kafka broker:
`export KAFKA_OPTS="-Djava.security.auth.login.config=/Users/workspace/kafka/kafka_server_jaas.conf`
Start kafka broker:
`kafka-server-start /usr/local/etc/kafka/server.properties`