Configure available processors for tests

[mosche]

Unless configured otherwise (by means of tests.configure_test_clusters_with_one_processor) tests should be configured to run with a small number of processors (2 was considered a good number by Core/Infra) to adequately size thread pools in relation to the available heap, e.g. using the node.processors setting or -XX:ActiveProcessorCount alternatively.

Sizing thread pools properly is necessary to prevent OOMs when attempting to allocate thread local direct buffers as done in various places.

[elasticsearchmachine]

Pinging @elastic/es-delivery (Team:Delivery)

[mark-vieira]

adequately size thread pools in relation to the available heap, e.g. using the node.processors setting or -XX:ActiveProcessorCount alternatively.

For added context, by default test cluster nodes use a relatively small heap of 512mb by default so when running many of these concurrently on a system with lots of CPU cores (like in CI) you end up with disproportionately large thread pools.