Draft: Initial NoCloud Support (and other Miscellany)

DO NOT MERGE - this branch got way too big, need to separate this into smaller commits/MRs.

clouds/

• import & register nocloud
• AWS: support optional encrypted images (based on !139 (closed))
• add client() to adapter (for clouds that lack session)
• generalize and split image_config() into CloudAdapterInterface
• add image_upload() and release_image() into CloudAdapterInterface
  • for uploading images for direct download and making them available for download, independent of cloud
  • image_upload() after successful image_import() (which for NoCloud is a NOOP)
  • release_image() after successful image_publish() (which for NoCloud is a NOOP)

configs/

• add nocloud configs
• general tweaks and refinements

scripts/

• nocloud cloud-init datasource

image_configs.py

• allow EXCLUDE/end_of_life to be defined inside WHEN blocks

gen_mksite_releases.py

• generates mustache-ready alpine/infra/alpine-mksite/clouds/releases.yaml for new alpinelinux.org/clouds page