Retrieve cluster info in serverless public mode

[gbanasiak]

#1785 bypassed client.wait_for_rest_layer() call in driver.Driver.prepare_benchmark() but unnecessarily also bypassed retrieve_cluster_info() method. This PR fixes this.

Before the change:

% esrally race --track=geonames --target-hosts=${ES_HOST}:443 --pipeline=benchmark-only --client-options="use_ssl:true,api_key:${ES_API_KEY}" --on-error=abort --test-mode                                      
[..]
[INFO] Race id is [b109fef1-5f31-4e63-92c5-dc3a6a10c3a1]
[INFO] Detected Elasticsearch Serverless mode with operator=[False].
[INFO] Excluding [check-cluster-health], [force-merge], [wait-until-merges-finish], [index-stats], [node-stats] as challenge [append-no-conflicts] is run on serverless.
[INFO] Racing on track [geonames], challenge [append-no-conflicts] and car ['external'] with version [oss]. <--- HERE

After:

% esrally race --track=geonames --target-hosts=${ES_HOST}:443 --pipeline=benchmark-only --client-options="use_ssl:true,api_key:${ES_API_KEY}" --on-error=abort --test-mode
[..]
[INFO] Race id is [b7a2f8a9-8c58-4690-aaa2-0d2424ee7f37]
[INFO] Detected Elasticsearch Serverless mode with operator=[False].
[INFO] Excluding [check-cluster-health], [force-merge], [wait-until-merges-finish], [index-stats], [node-stats] as challenge [append-no-conflicts] is run on serverless.
[INFO] Racing on track [geonames], challenge [append-no-conflicts] and car ['external'] with version [8.11.0]. <--- HERE

[b-deam]

Good catch.