alpine-cloud-images, part three

This third "initial" MR includes the script and configuration changes that were necessary for releasing Alpine Linux 3.15.0. In summary...

• combining configs, actions, and artifact YAML into a single images.yaml
• rename determine_actions() to refresh_state() - and do this after Packer has completed
• refine/flatten image config attributes
• move testing configs to overlays/testing/, which can be applied via --custom
• separate pip upgrade of pip and add pip install of wheel (which helps install of other libs)
• intermediate DictObj(dict) class that Tags class extends
• add configs for 3.15; 3.15 & edge aarch64 needed additional module config - and extend setup to ensure mkinitfs works
• fixes from MR !127 (merged)

There are a number of TODO items flagged here that were encountered during the release, which we may address in separate issues/MRs:

• get EOL for versions from an authoritative source (instead of in config file)
• investigate/fix additional "null" region key in deployed artifacts. (done)