dokku_image: Detect unchanged docker images

[blockloop]

I believe this fixes #156
Happy to discuss

[blockloop]

Temporary work around

- name: create app
  failed_when: "'error' in image_pull.meta and 'No changes detected, skipping git commit' not in image_pull.meta.error"
  register: image_pull
  dokku_image:
    app: &app nginx
    image: docker.io/nginx

[ltalirz]

This looks good to me, thanks a lot for the fix!