cylc vip (validate-install-play)

[wxtim]

#3896 - This PR draft, but currently implements the validate install play working practice. I think that it works in principle for the other working practices.

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.
• Tests are included (or explain why tests are not needed).
• 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.

[hjoliver]

(Looking forward to this - I am now gagging for cylc vip myself 😁 )

[wxtim]

@hjoliver @datamel - I have undrafted this PR - the functionality is ready for review. I'm going to have a look at whether I can add labels to the help output indicating which commands each option applies to - i.e.

-    --workflow-name -n Some info
+    --workflow-name -n [cylc install] Some info

However, I don't see any reason this couldn't be follow in if you wanted to focus on getting this in.

[datamel]

A few comments on the help docs to register while I get on with more thorough manual testing.
I have tested this with a basic workflow, no options just yet, all is working really well 🎉