Adding old-image removal capability.

The docker_pull module will now keep the 3 most recent images of
whichever image is being pulled and remove the older versions.
'keep_images' can be modified to increase or decrease the number of
images to keep, the default is set to 3.
Add documentation to docker_pull module.
Add option to keep_images to prevent deletion of images.

Documentation updated to reflect this change. Providing the value '-1'
to 'keep_images' opts out of image removal.
Image removal updated to be opt-in.
Splitting the guts of '_get_old_images' out.

Guts are now in '_image_ids_for_removal', a stand-alone function instead
of a method because testing a method involves dealing with AnsibleModule
which in turn involves dealing with Ansible magic. Attempted that, not
worth the effort.
Missing 'if __name__' thing.

This is apparently important and makes the AnsibleModule import deal go
away. Moving guts back to where they were.
Testing guts of 'remove_old_images', '_image_ids_for_removal'.

'_image_ids_for_removal' parses the output of 'docker images' and is
what returns the list of image ids for deletion. So that's what gets
tested.
Update runtest with python test script.
Updating python test script, forgot things.
Minor comments added.
Replace copy-pasta class name.
Ignoring module bytecode
Merge branch 'vky-remove-old-images'