Exit with an error if an image fails to build
Exit with an error if an image fails to build

Use subprocess.check_call when images are being built so failures
are detected and the client exits. Without this users (or scripts)
don't notice an error when it is happening as tripleclient just
continued and built the next image in the list.

Change-Id: Ie528fdd63b636761574b7fc9aaf86623799305a9

openstack/python-tripleoclient