Include static service levels in service levels listing

[Jadw1]

There was a request https://github.com/scylladb/scylla-enterprise/issues/814 to include static service levels in statements LIST SERVICE_LEVEL x and LIST ALL SERVICE_LEVELS. However I have a concern whether this would be a good change and because this is only a cosmetic change, I'm extracting it to a separate issue to close the mentioned one.

Arguments for:

• in enterprise version, default service level (which is a static one) determines shares (1000) of scheduling group for users w/o attached service levels

Arguments against:

• static service levels are only used in enterprise and only for scheduling groups. They have no affect on client_state's timeout and workload type
• there is only one static service level (the default one) and AFAIK it cannot be altered or so
• at least for me, it might be confusing to list that SLs when user cannot alter them or attach them to any role

[Jadw1]

cc @tzach

[piodul]

@tzach ping, could you answer @Jadw1's concern? We are not sure whether we should work on this or close the issue.