KAFKA-5227: Remove unsafe assertion, make jaas config safer

[rajinisivaram]

No description provided.

[rajinisivaram]

@ijuma Can you take a look? Thank you...

The assertion failure in https://builds.apache.org/job/kafka-trunk-jdk7/2176 could be because of the small timing window where the controller thread was shutdown and it released the latch, but the main thread did the check for the thread before the thread itself was terminated. So I have removed the assertion in ZooKeeperTestHarness that I added in the other PR. The failure in KAFKA-5227 did not fail the thread assertion, so it wasn't caused by brokers.

I reversed the ordering of setting the System property and resetting Configuration to avoid other threads (which shouldn't be there) loading Configuration again before the System property is set.

[rajinisivaram]

There are tests which leave ZooKeeper clients behind. And the client SendThread loads Configuration. Hopefully this PR will fix the issue without having to track down every thread leak.

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/kafka-pr-jdk8-scala2.12/3833/
Test PASSed (JDK 8 and Scala 2.12).

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/kafka-pr-jdk7-scala2.11/3845/
Test PASSed (JDK 7 and Scala 2.11).

[ijuma]

Thanks, LGTM. We probably still need to track down tests that are leaking ZK instances as such leaks tend to cause test instability as well.

JaasContextTest also needed to be updated in a similar way as other tests and I'll do that before merging to trunk.