Unblock percolator, so, and sql tracks for public serverless

[gbanasiak]

Similar to #465.

Remarks:

• I've added specific index selection in operations used by so frequent items challenge. That was required to avoid errors in serverless project which never comes empty, i.e. includes indices other than the indices added by the challenge.
• The so frequent items requires post_ingest_sleep_duration high enough for refresh to occur. The default refresh interval is 15s in serverless, the default post ingest sleep duration is 30s, so defaults work.
• The sql track does not support test mode, so I've tested with the following track params (based on https://github.com/elastic/rally-tracks/tree/master/sql#testing):

--track-params="ingest_percentage:1,query_percentage:2,post_ingest_sleep:true"

[ebadyano]

LGTM, thank you