config: make defaults explicit

[oliver-sanders]

There are a few configurations for which the default values are not properly centralised in the parsec SPEC. We should have a run through the SPEC's and ensure all defaults are defined in the spec (note they are auto-documented when defined here).

Here's an example of a hardcoded default:

https://github.com/cylc/cylc-flow/blob/9549dbd7e7fb7e2cd61589552cf1ff55dde414b8/cylc/flow/task_events_mgr.py#L1512

Pull requests welcome!

[dpmatthews]

Several addressed by #4848