Add warning when benchmarking multiple target hosts

[OVI3D0]

Description

Adds a warning when benchmarking multiple target hosts. Because OSB considers node count when checking for cluster health, two single-node clusters passed in via comma separated strings in the --target-hosts flag will result in an unhealthy status, and the benchmark will hang. However, as @IanHoang pointed out, passing in the two clusters via a JSON file will allow this check to pass.

Here is an example of the warning:

dev-dsk-mikeovi-2b-40a6f422 % opensearch-benchmark execute-test --target-hosts=localhost:9200,localhost:3200 --workload=geonames --test-mode --kill-running-processes

   ____                  _____                      __       ____                  __                         __
  / __ \____  ___  ____ / ___/___  ____ ___________/ /_     / __ )___  ____  _____/ /_  ____ ___  ____ ______/ /__
 / / / / __ \/ _ \/ __ \\__ \/ _ \/ __ `/ ___/ ___/ __ \   / __  / _ \/ __ \/ ___/ __ \/ __ `__ \/ __ `/ ___/ //_/
/ /_/ / /_/ /  __/ / / /__/ /  __/ /_/ / /  / /__/ / / /  / /_/ /  __/ / / / /__/ / / / / / / / / /_/ / /  / ,<
\____/ .___/\___/_/ /_/____/\___/\__,_/_/   \___/_/ /_/  /_____/\___/_/ /_/\___/_/ /_/_/ /_/ /_/\__,_/_/  /_/|_|
    /_/

[INFO] [Test Execution ID]: e2f9c79f-47a9-4268-8bf4-b285182b4ac0
[WARNING] WARNING: Benchmark runs with multiple target hosts should be passed in as a JSON file such as:
{
  "default": [
    {"host": "127.0.0.1", "port": 9200} # Specify nodes for cluster 1
  ],
  "remote":[
    {"host": "10.127.0.3", "port": 9200} # Specify nodes for cluster 2
  ]
}

Issues Resolved

#730

Testing

• New functionality includes testing

make test + make it

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.