Publish GCP images with allUsers visibility

[MrBatschner]

What would you like to be added:

Garden Linux images on GCP are published with allAuthenticatedUsers permissions. This has to be changed to allUsers (also retroactively applied to all existing images) so that it is possible to list and consume the images for users authenticated through a federated IdP.

See the clear documentation at https://cloud.google.com/iam/docs/overview#all-authenticated-users.

Why is this needed:

Gardener will use IdP access to cloud platforms for future clusters which should still be able to consume Garden Linux image.

[MrBatschner]

Relevant code that needs to be adapted is here:

glci/glci/gcp.py

Lines 150 to 158 in 2442377

	iam_policies = images.getIamPolicy(
	project=gcp_project_name, resource=image_name
	).execute()
	if not 'bindings' in iam_policies:
	iam_policies = []
	iam_policies.append({
	'members': ['allAuthenticatedUsers'],
	'role': 'roles/compute.imageUser',
	})