docker: Adds docker images utils.

Usage:
  d = images.DockerImages(self)

  # list all images
  all_images = d.list_imgs()

  # list all full_names
  full_names = d.list_imgs_full_name()

  # filter images based on full_name
  images_with_fn = d.list_imgs_with_full_name("fedora:latest")

  # filter already created list (helper)
  filter_list_by_components(image_list, repo=..., repo_addr=...,
                            user=..., tag=...)

Signed-off-by: Jiří Župka <jzupka@redhat.com>
docker: Adds sample test for docker images.

Signed-off-by: Jiří Župka <jzupka@redhat.com>