change to new mirror, dry kafka version, better curl flags

[jsvd]

Three concerns are mixed in this PR, I can split them into separate PRs if the reviewer so wishes:

1. change the mirror from where to download kafka for testing
2. DRY the kafka version location to ensure tests run with the same kafka broker version as the client
3. use a better set of curl flags to maintain error messages and still avoid flooding ci logs with download progress

[jsvd]

@kaisecheng since you're deep into the kafka plugin code, maybe you can review this? thanks

[kaisecheng]

@jsvd kafka_test_setup.sh has defined KAFKA_VERSION and CONFLUENT_VERSION as well. Sometimes, I run the kafka_test_setup directly. It will be great if you can bring the gradle version to the script.

Once the conflicts are resolved, I can approve.