KAFKA-21073: Enable SASL for the native image docker examples and sanity test

[muralibasani]

Ref : https://issues.apache.org/jira/browse/KAFKA-21073

• Remove the three stale "SASL not supported on native" notes from docker/examples/README.md and add the apache/kafka-native run command to each SASL example, for parity with the other examples.
• Remove the MODE == "jvm" guard in docker_sanity_test.py so the SASL flow runs for the native image as well as the JVM image.

Reviewers: Chia-Ping Tsai chia7712@gmail.com

[muralibasani]

Tests locally

% python3 -m pip install -r docker/requirements.txt

% python3 docker/docker_build_test.py kafka-native-sanity -tag test -type native \
    --kafka-archive "$(pwd)/core/build/distributions/kafka_2.13-4.5.0-SNAPSHOT.tgz" -t

Running SASL Flow Tests
Created topic test-topic-sasl.
The consumer rebalance protocol (KIP-848) is production-ready! Set group.protocol=consumer to try it out. See https://kafka.apache.org/documentation/#consumer_rebalance_protocol
Processed a total of 1 messages

[chia7712]

It seems the MODE is useless :)

[muralibasani]

Ah yes, removed it.

[muralibasani]

Thank you for the review.