Clear Template Variables from scheduler options before reload

[wxtim]

Closes #5968

Warning

Branch is based on #5868 and should not be merged before that. Merged.

This passes the manual test described in #5968, and the equivelent test with -D.

I've tried to integration and functional tests, but have hit assorted walls:

• Functional relies on Rose and Cylc-Rose installs, it might be possible to fake a plugin for the test, but I struggled to make it work with Rose.
• Integration makes it easier to mock the plugin, but I've struggled to mock the moving parts required without making the test a meaningless cookie-cutter inversion of the logic.
• Given that this is about CLI interaction Functional tests make more sense IMO.

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).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests - See explanations above.
• CHANGES.md entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[oliver-sanders]

(note, the issue is against 8.2.x)