MINOR: ensure original use of prop_file in verifiable producer
MINOR: ensure original use of prop_file in verifiable producer

This PR: https://github.com/apache/kafka/pull/958 fixed the use of prop_file in the situation when we have multiple producers (before, every producer will add to the config). However, it assumes that self.prop_file is initially "". This is correct for all existing tests, but it precludes us from extending verifiable producer and adding more properties to the producer config (same as console consumer).

This is a small PR to change the behavior to the original, but also make verifiable producer use prop_file method to be consistent with console consumer.

Also few more fixes to verifiable producer came up during the review:
-- fixed each_produced_at_least() method
-- more straightforward use of compression types

granders please review.

Author: Anna Povzner <anna@confluent.io>

Reviewers: Geoff Anderson <geoff@confluent.io>, Ewen Cheslack-Postava <ewen@confluent.io>

Closes #1192 from apovzner/fix_verifiable_producer

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