When a image without a registry is provided as input, i.e.
atomic pull busybox
We are not able to resolve that reference now because we can
no longer depend on docker's algo's for that as we now
use skopeo. Therefore, we now take the input, see if skopeo
inspect can resolve it; and then decompose the fqdn so we
can check if the registry is secure. Had to make a small
change on the return types of util.decompose to ensure
it returns str objects or else it will return unicode in
python2.
Closes: #650
Approved by: rhatdan
c20cc24d06