config: support the variant field

[nalind]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Add the variant field, along with methods for setting and querying it, and expose them in the buildah config and buildah inspect commands.

When setting an initial architecture for a container based on an image which doesn't contain an architecture, or from "scratch", normalize the architecture name we've been given, and set both it and the variant field at the same time.

Provide normalized architecture+variant values in buildah info.

How to verify it

New integration test!

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

Does this PR introduce a user-facing change?

None

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nalind

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[flouthoc]

LGTM

[rhatdan]

Could you add a test for buildah config --variant ...

[nalind]

Could you add a test for buildah config --variant ...

Added.

[rhatdan]

I don't see --variant added?
I was thinking of

buildah from scratch
buildah add foobar
buildah config --arch XYZ --variant foobar
buildah commit
buildah inspect.

[nalind]

I don't see --variant added? I was thinking of

buildah from scratch buildah add foobar buildah config --arch XYZ --variant foobar buildah commit buildah inspect.

The "config" test in config.bats does that for both OCI and Docker formats, so I added it there as another option/result combination that it checks.

[rhatdan]

OK.
/lgtm