Before this commit, when the child image affected by CVE was rebuilt
and its parent image was not affected by the CVE, the parent image
was kept in the original NVR.

This is unfortunate behavior, because the original NVR of parent image
can contain CVE which is already fixed in latest released release of
the parent image.

In this commit, Freshmaker tries to find out the latest released
parent container image and use it instead of the original NVR.

It does it in very safe way which won't covert 100% of cases, but it
is good enough to be used as a first step to cover majority of cases.

The commit covers only following case:

• The parent image must have the auto_rebuild_tags set in its container
  repository. This is needed in order to find out the available tags
  in container repository with multiple tags. In case the parent image
  does not have auto_rebuild_tags set, the original parent NVR is used.
• The latest parent image must have the same Name and Version as
  the original parent NVR. This prevents upgrades for example from
  python-2.7-1 parent container image to python-3.3-1 parent container image.
• The latest parent image must appear in the same container repository as
  the original parent image.