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.