Marketplace: test/validate arm template

[vdice]

We could use better ARM template testing/validation. Ref #37 -- apparently our current arm-ttk based verification workflow doesn't catch some aspects.

Options:

Full e2e deploy

With az creds, create a deployment from the template, e.g.:
az deployment group create --name test-deployment --resource-group test-rg --template-file mainTemplate.json

Note: we would need to supply correct values for the placeholder variables here: https://github.com/spinkube/azure/blob/main/marketplace/mainTemplate.json#L133-L137

Just validation?

I was hopeful use of az deployment group validate might cover us here but in my cursory test with trying to catch the issue mentioned in #37, it didn't seem to produce the expected error (eg 'Basic' is invalid for the SKU).

Revisit this to see if we can catch errors using this approach -- it would be lighterweight and we wouldn't need to deploy resources. Though, perhaps we'll want the full e2e at some point anyways.