fix(performance): add sleep after warm-up

[juliayakovlev]

Add 2 minutes sleep after warm-up and before step start in predefined steps performnace test. This is for let to all background processes to finish. It may have the impact especially in tablet test - to complete tablets rebuild

Fixes: #9852

Testing

• [ ]

PR pre-checks (self review)

• I added the relevant backport labels
• I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)
• Add unit tests to cover my changes (under unit-test/ folder)
• Update the Readme/doc folder relevant to this change (if needed)

[fruch]

Can you share a graph from a run using this code, to see if 2 min is enough time to be able to clearly see warmup doesn't have affect on the main load

[juliayakovlev]

https://argus.scylladb.com/tests/scylla-cluster-tests/9df777c1-3b2a-42ed-8846-dd7559d5efd0

This run (with tablets) with sleep 120 sec after warm-up and before 150K step. the latency is 1.75ms

	P90 read [ms]	P99 read [ms]	Throughput read [op/s]	duration [HH:MM:SS]	start time	Overview	QA dashboard
Cycle #​1	$${\color{green}0.41}$$	$${\color{green}1.75}$$	149,719	00:30:15	10:45:11	view	view

[juliayakovlev]

https://argus.scylladb.com/tests/scylla-cluster-tests/803ec3ab-91ec-441a-b324-f45b6cb6ba1c

This run (with tablets) with sleep 240 sec after warm-up and before 150K step. The latency is 0.48

	P90 read [ms]	P99 read [ms]	Throughput read [op/s]	duration [HH:MM:SS]	start time	Overview	QA dashboard
Cycle #​1	$${\color{green}0.41}$$	$${\color{green}0.48}$$	149,645	00:30:15	13:44:32	view	view

[roydahan]

Let's stop wasting more money on runs.

[fruch]

LGTM