CPU utilization on "Overview" should only include scheduling classes that are involved in queries processing

[vladzcloudius]

System information

• Scylla version (you are using): irrelevant
• Are you willing to contribute it (Yes/No): No

Describe the feature and the current behavior/state.
Instead of showing the total seastar's reactor CPU utilization (which may be confusing), show only CPU usage of all sl:... and commitlog scheduling groups.
These represent the actual "good put" of the system.

Who will benefit with this feature?
Customers