Unblock elastic/logs for public serverless

[gbanasiak]

Similar to #465.

Remarks:

• unblocking limited to logging-indexing, logging-querying and logging-indexing-querying challenges which are applicable to serverless today,
• shared custom index settings moved to a dedicated component template (track-custom-shared-settings) to avoid repetition, this makes @custom templates empty, but these are kept as placeholders for any future changes,
• there is no explicit force merge step in elastic/logs so optional post-ingest sleep was not introduced,
• compression-statistics custom runner had to be excluded at the track level as it is based on index stats.

Tests:

TRACK=elastic/logs
esrally race --track-path=../rally-tracks/$TRACK --target-hosts=${ES_HOST}:443 --pipeline=benchmark-only --client-options="use_ssl:true,api_key:${ES_API_KEY}" --on-error=abort --challenge="logging-indexing" --test-mode --track-params="start_date:2021-01-01T00-00-00Z,end_date:2021-01-01T00-10-00Z"
esrally race --track-path=../rally-tracks/$TRACK --target-hosts=${ES_HOST}:443 --pipeline=benchmark-only --client-options="use_ssl:true,api_key:${ES_API_KEY}" --on-error=abort --challenge="logging-indexing" --test-mode --track-params="start_date:2021-01-01T00-00-00Z,end_date:2021-01-01T00-10-00Z,refresh_interval:17s"
esrally race --track-path=../rally-tracks/$TRACK --target-hosts=${ES_HOST}:443 --pipeline=benchmark-only --client-options="use_ssl:true,api_key:${ES_API_KEY}" --on-error=abort --challenge="logging-querying" --test-mode --exclude-tasks="tag:setup" --track-params="query_warmup_time_period:1,query_time_period:1,workflow_time_interval:1,think_time_interval:1"
esrally race --track-path=../rally-tracks/$TRACK --target-hosts=${ES_HOST}:443 --pipeline=benchmark-only --client-options="use_ssl:true,api_key:${ES_API_KEY}" --on-error=abort --challenge="logging-indexing-querying" --test-mode --track-params="start_date:2021-01-01T00-00-00Z,end_date:2021-01-01T00-10-00Z,query_warmup_time_period:1,query_time_period:1,workflow_time_interval:1,think_time_interval:1"

[gareth-ellis]

LGTM

[inqueue]

LGTM, nice work