container_image_list can be a string

[edenhaus]

Until now container_image_list had to be a list as it is required for the loop parameter.

with_items works with string and list.

After this PR the following is possible and handy if use are using the role for a single image:

container_image_list: "hello-world" # Or any other image

But it is still possible

container_image_list: 
  - "hello-world" # Or any other image

Or it is also the following possible, but then the relevant task is skipped

container_image_list: 
  - image: "hello-world" # Or any other image

[ikke-t]

Thanks!