[TEST] randomly added node.bench=true to client node in test cluster and re-enabled REST benchmark tests based on number of bench nodes available

In our REST tests we already have support for features and skip sections that allow to skip tests if a feature is not supported.
We can then add a skip section based on the benchmark feature to the benchmark tests and execute them only when they are supported, knowing that they need at least a node with node.bench settings within the cluster. We can check that this requirement is met by calling the nodes info api.

This way we can dynamically decide whether to execute those tests or not and we don't need to have a node.bench around all the time. In fact, given that the REST tests use the GLOBAL cluster, we want to be able to randomize settings as much as possible and run tests against default settings as well. Also, this mechanism can be easily supported by the external cluster implementation that is used during the release process.

Introduced ability to disable benchmark nodes which is needed by BenchmarkNegativeTest.