Check third-party container image metadata.
Check third-party container image metadata.

During the evaluation of a third parties image that doesn't
contain the expected metadata json, the image_uploader section
fails.
This commit adds the check on the returned dict and adds the
ability to use the fallback_tag.

Change-Id: I1809400d798d4350f9642a348ecbe81714e6af47
Closes-Bug: #1834456

openstack/tripleo-common,openstack/tripleo-common