cosa import: Do not fail on non-existing `com.coreos.osname` label

[travier]

We add the com.coreos.osname label to our image in coreos/fedora-coreos-config#3584 but all our previous images don't have it thus running cosa import on them fails.

Let's default to fedora-coreos or add a command line option to set it for older images.

[travier]

Previous discussion in #4164 (comment). The argument would be optional, used only if the container does not have a label already.

[jlebon]

Not strongly against, though in what context do you need to do this?

[travier]

I'm building disk images from "older" (current FCOS stable) derived container images imported using cosa import and this failed on this. For now I'm adding a label to the container image but we might hit the same issue once we start using that flow to work on "older" RHCOS that don't have the label either.

[dustymabe]

add a command line option to set it for older images.

I'm happy with this as an option.

I'm not sure I'd be happy with a default value since there are some things that are triggered later that have different behavior based on osname, and if you're using the wrong one it could lead to unintended outcomes.