Mantle: use Butane libs

[darkmuggle]

Changes:

• vendor in Butane
• added helper function
• change qemuexec to use Butane libs over FCCT

While looking at some tests, I noticed a pattern of:

	/* The FCCT config used for generating this ignition config:
	variant: fcos
	version: 1.3.0
	boot_device:
	  mirror:
	    devices:
	      - /dev/sda
	      - /dev/sdb
	      - /dev/sdc
	*/
	bootmirror = <long json>

This change will allow developers the option to use YAML through Butane or write the long JSON.

[cgwalters]

Did you have a specific reason to switch to vendoring vs forking the binary? (Commit message doesn't say) Currently we don't have dependabot set up on this repo, which is mostly OK but butane is something we will care about and want to keep updated. Right now we have implicit updates when the Fedora package is built. (Also, because not all tooling in this repository is in Go, we need to keep butane as a binary anyways)

But I'm OK with this too!

[bgilbert]

👍, APIs >> subprocesses. We can add "update Mantle vendoring" to the Butane release checklist, as we currently have with Ignition.

But let's properly integrate this into platform/conf, rather than adding a box-on-the-side internal API. Let me dig into this a bit, it shouldn't be hard to do.

[cgwalters]

+1, APIs >> subprocesses.

Yes, as a general rule. But I think my point stands that this specific repository is gluing together a lot of different tools in different languages - e.g. we also fork rpm-ostree and parse its JSON, that's unlikely to change. And I definitely run butane on the CLI too, so it will be surprising if the versions differ.

(Anyways not arguing against this either to be clear)

[bgilbert]

#2246 integrates Butane support into platform/conf and updates the affected tests.

[darkmuggle]

Superseded by #2246, which is a superior solution. Closing.