[TEST] run REST tests against multiple nodes (round-robin)

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
[TEST] removed leftover delete alias test in delete mapping
[TEST] slightly sped up warmer tests by providing warmers during index creation
[TEST] disabled refresh for get_source realtime test