[api_tests]: cluster.rebalance should assert if rebalance succeed.

We recently added rebalance function in cluster.py - which invokes a rebalance via cluster_run_lib.wait_for_rebalance where we check if the status for the rebalance isn't running - https://src.couchbase.org/source/xref/elixir/ns_server/pylib/cluster_run_lib.py?r=ac333a0b#787

It would be nicer if we could assert rebalance succeeded - currently we return rebalance has completed even if the status is failed.