Act more like aws cli / conducive to CI

[aidansteele]

aws cloudformation package and aws cloudformation deploy behave differently to stackit package and stackit execute.

At first I thought it would be more useful to create and display a changeset as part of stackit package, making it similar to terraform plan. This arguably is more useful in the typical case, but isn't as useful in the CI case.

Specifically, in a CI setup we might want to archive "artifacts" from a Build stage and use them in a later Deploy stage. Right now we are archiving stackit.packaged.yml. The problem is that this isn't replayable, a user can't re-run just the Deploy stage as they might expect.

We could ameliorate this with the following design:

• stackit package acts identically to aws cloudformation package, albeit with sensible defaults -- it will emit a <template name>.packaged.yml file.
• stackit plan is analogous to terraform plan
• stackit apply is analogous to terraform apply
• stackit up --stack-name <stack> --template <template name>.packaged.yml is a wrapper around plan && apply, i.e. it creates and executes a change set.

[aidansteele]

Actually, there's no reason up shouldn't be a wrapper around package, plan and apply.

[aidansteele]

As of eb43d0b

• up is done. It now packages, plans and applies.
• package behaves as described above.
• plan and apply don't exist yet