Global config changes: process pool, logging

[MetRonnie]

These changes partially address #3696

• process pool * -> [scheduler]process pool *
• [suite logging] -> [scheduler][logging]
• run directory rolling archive length -> [scheduler]run directory rolling archive length (Update: this setting was later removed - Remove run dir housekeeping #4162)

Also the test_header function create_test_global_config has been simplified to only arg which is appended to the user's global config, as there are no longer any top-level global config items that would need to be prepended.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Already covered by existing tests.
• No changelog entry needed (entry will be written after conf: ensure all proposed changes have been enacted #3696 is closed)
• No documentation update required (docs will be updated after conf: ensure all proposed changes have been enacted #3696 is closed)
• No dependency changes.

[hjoliver]

👍