ostree: add convenience function for using default OSTree deployment

[dustymabe]

This adds a default: true option for all cases where OSTree information is specified in schemas and allows for the information to be picked up from the filesystem.

This is a safe operation because when building disk images there is no known case where having two deployments makes sense. In the case there ever were a case then the osname, ref, and serial options still exist and can be used.

[dustymabe]

This obsoletes #1527

[dustymabe]

ok over in #1527 (comment) we came up with something like this as a proposal:

So the concensus is something like OneOf:

"deployment": {
  "osname": ...,
  "ref": ...
  "serial": ... (default 0)
}

or

"default-deployment": true (default false)

When implementing that we thought it would be more appropriate for the new option to be under deployment and not a new top level property. So that is what we came up with here where we now allow the user to specify something like:

"deployment": {
  "default": true
}

Since this was a change from what we talked about before I figured it'd be worth calling out and asking if that is an acceptable path forward.

[dustymabe]

ok rebased and did a new push. Hopefully CI passes now.

[dustymabe]

rebased and fixed the merge conflict!

[achilleas-k]

Since the idea behind this PR was discussed in a previous one (#1527 (comment)) and I know others have thoughts on the value of being explicit, I'm going to ask for more people so sign off on this change.
@thozza @bcl @mvo5

EDIT: Consider this an approval from me.

[thozza]

LGTM. 👍

[mvo5]

This looks fine to me. I have some suggestions inline but it's all nitpicks or ideas that are totally optional. I have no strong opinion about the feature itself. It seems fine to me to infer some related things from the tree (but I think it does technically violate some of the principles in #1568). OTOH these principles are not set in stone and I think we have other cases where things are inferred.

I attached a small patch with my suggestions and small tests as I was playing around a bit with the code but it can be a) a new PR or b) ignored as it's just tweaks.

0001-ostree-small-tweaks-for-parse_deployment_option.patch.txt

[dustymabe]

I attached a small patch with my suggestions and small tests as I was playing around a bit with the code but it can be a) a new PR or b) ignored as it's just tweaks.

0001-ostree-small-tweaks-for-parse_deployment_option.patch.txt

I applied this patch and marked you as Co-Author of the commit!

[mvo5]

Thank you!