Make docker_image.force_source configurable

[andygrunwald]

Each ansible execution, this task is marked as changed.
In my usecase, I don't want to have docker_image.force_source enforced.

This PR makes this setting configurable.
The change is backwards compatible.

[mhutter]

What's the purpose of this feature? If you do not want to force pulls, just disable container_docker_pull. The image will still be pulled when the container is started.

[andygrunwald]

The usecase would be:

• During ansible execution you want that the image is being pulled, but not if the image is already present at the machine (I am using version tags and not :latest).
• I want to ensure that the image is always present BEFORE systems is starting the service

On top of this:
If I want to execute this via CI to ensure changes like "If I would execute this, will there be a change?", this force setting would always say "Who knows?"

[mhutter]

Ah I see, makes sense