Use community_images

[sjpb]

Uses the azimuth_cloud.azimuth_ops.community_images role to simplify provision and use of StackHPC images.

Significant changes:

• New playbook ansible/adhoc/image-pull.yml will idempotently download/convert/upload the relevant StackHPC image to the current cloud. Images are defined by Ansible community_images* variables. Note Ansible variable appliances_distro_version defaults to RL9 and must be overridden for RockyLinux 8 clusters. TODO: should I change this name??
• OpenTofu configuration now uses keys from community_images variable to specify cluster and nodegroup images. OpenTofu variable cluster_image_id replaced by cluster_image_key and nodegroup parameter image_id replaced by image_key.
• Packer image build configuration also uses keys from community_image variable to specifiy build's source image. Packer variable source_image_id replaced with source_image_key. TODO: think about how this works for multi-stage builds.
• Both OpenTofu and Packer default to using the stackhpc image key, meaning the appropriate StackHPC image is the default for both clusters and for site-specific image builds. Running the image-pull.yml playbook will if necessary update the image. TODO: think about how to invalidate current site builds when this happens??

TODO: Integrate community_images into CI and describe changes
TODO: docs
TODO: figure out how to get site builds to add to cluster_image_ids.