downloads: split file and containers (#13581)

Splits the definitions into two dicts, while joining then for now to
preserve the downloads dict.

Update all consumers of the variables, which are roughly:
- manual includes of the download role (which will be removed in the
  short future)
- install tasks using the resolved 'dest' to find the artefacts

This will make further division of the downloads role easier.

contrib/offline/generate_list.sh

@@ -14,7 +14,7 @@ grep 'download_url:' ${REPO_ROOT_DIR}/${DOWNLOAD_YML} \
     | sed 's/^.*_url: //g;s/\"//g' > ${TEMP_DIR}/files.list.template
 
 # generate all images list template
-sed -n '/^downloads:/,/download_defaults:/p' ${REPO_ROOT_DIR}/${DOWNLOAD_YML} \
+sed -n '/^downloads_container:/,/download_defaults:/p' ${REPO_ROOT_DIR}/${DOWNLOAD_YML} \
     | sed -n "s/image_name: //p" | tr -d ' ' \
     | tr ' ' ':' | sed 's/\"//g' > ${TEMP_DIR}/images.list.template