INTEXT-111: Allow PP for Kafka producer context

JIRA: https://jira.spring.io/browse/INTEXT-111

 - Remove hard coded names for bean names at the producer context parser
 - Fix and update tests
 - Remove the use of `BeanFactory` to get the producerConfigurations in
KakfaProducerContext and use setter to set the producerConfigurations
 - Add logic to send the message from producer configuration when there
is no header specified for the topic and single producer configuration
is used.

Add tests to verify placeholder values

Fix code review comment and formatting

Polishing code style.

Add `KafkaProducerContext#theProducerConfiguration` for single `producerConfigurations` entry to avoid iterators