Avoid duplicate topic creation

[rondagostino]

Return TOPIC_ALREADY_EXISTS if an attempt is made to create a topic with a name that already exists.

Appended a few lines to existing create-topic test to try creating the same topic a second time and confirming that we get the expected error; the test failed prior to the fix and succeeded with it. Augmented streams_smoke_test.py system test to use both Zookeeper and Raft quorums; the Raft quorum cases were flaky before the fix and reliably passed with the fix.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[cmccabe]

LGTM. Thanks, @rondagostino !