Clean up the example jmx file

[julianharty]

I discovered through a messy, time-consuming process that various changes and edits to the jmx file I was using hadn't reached the example jmx file in the project. Examples include:

• Missing out the Counter used by the consumer
• having two similar sounding variables topic and topic.name where the Producer was expecting topic, didn't find it, which caused errors being reported at runtime and the producer couldn't produce data as the topic name was invalid.
• discovering the ssl.truststore.password was specified twice in different sections of the test plan and I'd edited only one, not both.

All these highlight it's time to focus on improving the consistency and use of variables and parameters across the project. Here's where we'll track the changes and improvements.

[julianharty]

With various changes and updates leading to https://github.com/commercetest/pepper-box/commit/bd5aeef94af8500e1f478af5001e5ae487ea45d6 I think we've now got a fairly useful set of example test plans for jmeter with PepperBox and the Kafka Consumer script.