* "build" target is now "local"
* split ImageConfigManager and Tags (now ImageTags) out of image_config.py
* add ImageStorage class
* upload images & metadata if storage_url is defined
* "latest_build_image" method is now "get_latest_imported_tags"
* keep ImageConfig updated with latest status of import/publish/artifacts
* ImageConfig
  * some misc method renaming
  * add resovle_urls, upload_image, save_metadata, load_metadata
  * update refresh_state (still more TODO here)
* update README