This is 2 changes:
* Run dstat in unit test jobs so that we can get an idea of
* Remove the stestr concurrency cap. With 8 cores, we have
enough headroom to run the test dependencies (sql/zk) while
the tests are running too. Use all the CPU that's available.
This mirrors the configuration in Nodepool for using TLS-enabled
ZooKeeper in tests. We use the ensure-zookeeper role in order
to get a newer ZooKeeper than is supplied in bionic.