Fix docker_label keyword and fix label caps
Previously the key 'docker_labels' was used but this isn't actually
picked up by imagefactory because the parameter key it's looking for
is 'docker_label'. This patch resolves that.
Also included in this patch is a change to set the labels to be
lowercase as defined in the Fedora Container Guidelines.
fix the docker_label entry to be a valid JSON string
Signed-off-by: Adam Miller <maxamillion@fedoraproject.org>