Add unit tests for KStreamImpl

* Add Jenkinsfile that drives the Jenkins job.

* Start the downstream job.

* Minor updates of Jenkinsfile

* Set timeout to 4 hours.
* Only keep ce-trunk in the Kafka-Muckrake mapping

* Pass the same Kafka repo to the downstream test-cp job, and remove the
realJobPrefixes so that Jenkins can send out notifications.

* Fetch the Kafka branch name from the git repository.

* Use the env.CHANGE_BRANCH as the branch name for PR builds.

* Set the default muckrakeBranch to master.

* Pass PR-n as the branch name to the downstream build job.

* Trigger test-cp-downstream-builds-cekafka as the downstream job.

* Trim away the whitespaces from the kafkaRepo string.

List of commits from kafka-broker-plugins:

* Include all transitive dependencies in assembly (#71)
* Add OAuth LoginCallbackHandler for the Kafka server (#72)
* CPKAFKA-1593: Multi-tenant authorizer for CCloud (#66)
* CPKAFKA-1593: Integration tests for multi-tenant authorizer for CCloud (#67)
* CAAS-1960: Implement Phase 3.1 Metrics (#76)
* CAAS-1960: Add authenticated connections and authentication rate metrics per tenant (#77)
* Bump Confluent to 5.2.0-SNAPSHOT, Kafka to 2.2.0-SNAPSHOT
* Topic config constraints (#70)
* Remove redundant import in MultiTenantRequestContext (#79)
* 4.1.x to 5.0.x (#78)
* 5.0.x to 5.1.x (#82)
* CPKAFKA-1408: Add tenant throughput and request quota plugin (#68)
* Improve topic policy checks (#81)
* CPKAFKA-1410: Partition assignor to balance partitions at tenant level (#73)
* CPKAFKA-1694: Add partition throughput percentiles for monitoring (#84)