message: [backup] refine s3 sample config
  1. deal with the case where metadata folder doesn't exist when listing metadata files (otherwise for an empty backup storage, one need to manually create the folder)
  2. break commands to multiple lines and add comments
  Closes:
diff: [[env_vars]]
  -key = "BUCKET"
  +key = "BASE_PATH"
  value = "libra-backup"
  [commands]
  -create_backup = 'echo "$BACKUP_NAME"'
  -create_for_write = 'echo "s3://$BUCKET/$BACKUP_HANDLE/$FILE_NAME" && exec >&- && aws s3 cp - "s3://$BUCKET/$BACKUP_HANDLE/$FILE_NAME"'
  -open_for_read = 'aws s3 cp "$FILE_HANDLE" -'
  -save_metadata_line= 'aws s3 cp - "s3://$BUCKET/metadata/$FILE_NAME"'
  -list_metadata_files = 'aws s3 ls s3://$BUCKET/metadata/ | sed -ne "s/.* //p" | xargs -I{} echo s3://$BUCKET/metadata/{}'
  +create_backup = '''
  + echo "$BACKUP_NAME" # backup handle is the same with input backup name, output to stdout
  +'''
  +
  +create_for_write = '''
  + FILE_HANDLE="s3://$BASE_PATH/$BACKUP_HANDLE/$FILE_NAME" # file handle is the file name under the folder with the name of the backup handle
  + echo "$FILE_HANDLE" # output file handle to stdout
  + exec 1>&- # close stdout
  + aws s3 cp - "$FILE_HANDLE" # route stdin to file handle
  +'''
  +
  +open_for_read = '''
  + aws s3 cp "$FILE_HANDLE" - # route file handle content to stdout
  +'''
  +
  +save_metadata_line= '''
  + aws s3 cp - "s3://$BASE_PATH/metadata/$FILE_NAME" # save the line to a new file under the metadata folder
  +'''
  +
  +list_metadata_files = '''
  + (aws s3 ls s3://$BASE_PATH/metadata/ ||:) | sed -ne "s/.* //p" | xargs -I{} echo s3://$BASE_PATH/metadata/{} # list files under the metadata folder
  +'''
  