Run REST tests against multiple nodes

Multiple nodes are now started when running REST tests against the `TestCluster` (default randomized settings are now used instead of the hardcoded `1`)

Added also randomized round-robin based on all available nodes, and ability to provide multiple addresses when running tests against an external cluster to have the same behaviour.

Also sped up a few warmer tests and fixed get_source realtime test which might fail in some cases due to automatic refresh.