KAFKA-2526: command line --producer-property wins

[Jack Thomasson]

in https://github.com/apache/kafka/pull/16492 i propose a simple change 
to CLI kafka-console-producer that reorders value settings as follow:

1. default
2. config file by --producer.config
3. --producer-property
4. required
this fixes https://issues.apache.org/jira/browse/KAFKA-2526. values for 
key.serializer and value.serializer in particular are honored whereas 
previously they were overwritten due to incorrect order.


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