Broker-side cleanup.policy should be DEFAULT_CONFIG, not DYNAMIC_TOPIC_CONFIG

[twmb]

DYNAMIC_TOPIC_CONFIG is meant for when the default is changed. If I create a topic without a cleanup policy, the default cleanup policy is as configured on the broker (for me, cleanup.policy=delete). Describing a topic's configs shows:

cleanup.policy          delete      DYNAMIC_TOPIC_CONFIG

when it should show

cleanup.policy                           delete               DEFAULT_CONFIG

Also related to #2176, in a way.

JIRA Link: CORE-727

[twmb]

cc @michael-redpanda

[michael-redpanda]

Some analysis of cleanup.policy: https://github.com/redpanda-data/core-internal/issues/1066