ImageStorage and Class Refactoring

• "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)