Contact the notary server if ```--verify``` is specified (or `verify:
true` is enabled on the repo config) and verify if the image is signed,
use the returned value to pull the verified image.
Create a util sub cmd to add all utils that are handy for development
and already present in the luet codebase. We expose in this case `luet
util unpack` to unpack a docker image without a docker daemon running.
Several changes are included:
- Expose ensureDir in helpers, and call it in the Docker client. In
other implementations that was handled by CopyFile behind the scenes,
but that's not the case here
- Create accessor in Artifact to create Artifact objects from files.
This is handy when we have to carry over downloaded package content
into caches when artifacts are already verified
- Fix various issues around the imagePush flag, so now trees are pushed
forcefully each time
- Take into consideration the real artifact name when pushing single
files in the docker image. This behavior should be changed eventually,
because single files which aren't repository packages now are in its
own docker image, but we should have just one that brings the required
metadata alltogether.
