This is 2 changes:
* Run dstat in unit test jobs so that we can get an idea of
system performance/utilization.
* 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.
Change-Id: I9f250865f7043fdbb1fa8a01f1bc9508490accc1