images/capi/packer/azure: allow building images from various SIG images

[invidian]

What this PR does / why we need it:

As far as I understand, right now it is only possible to build images
from Marketplace images. This patch actually enables building from the
community images, directly shared images and private SIGs.

Signed-off-by: Mateusz Gozdek mgozdekof@gmail.com

If one wants to build Flatcar image using SIG, one should create packer.json file with content similar to below:

{
  "community_gallery_image_id": "/CommunityGalleries/<public galery name>/Images/flatcar-{{env `FLATCAR_CHANNEL`}}-amd64/Versions/{{env `FLATCAR_VERSION`}}",
  "image_offer": "",
  "image_publisher": "",
  "image_sku": "",
  "image_version": "",
  "plan_image_offer": "",
  "plan_image_publisher": "",
  "plan_image_sku": ""
}

For the time being though, let's keep building Flatcar from Marketplace image until we consider community galleries stable.

Additional context

Related to kubernetes-sigs/cluster-api-provider-azure#2890.

CC @jepio @johananl

[invidian]

/test pull-azure-sigs

[invidian]

This is ready for merging I think.

[jsturtevant]

Could add some docs to https://image-builder.sigs.k8s.io/capi/providers/azure.html on how to do this.

/lgtm

[invidian]

Could add some docs to https://image-builder.sigs.k8s.io/capi/providers/azure.html on how to do this.

Added some minimal docs. Let me know if you think there is a need for a whole section with some examples.

[invidian]

While this PR adds support for all combinations, Flatcar team will be using building from the private SIGs for the time being and resulting images will be published into community SIGs.

[kkeshavamurthy]

Thanks @invidian
/lgtm

[jsturtevant]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[invidian]

I didn't expect such a quick merge, thanks!