zun help pull
usage: zun pull -r <repo>
Pull an image.
Optional arguments:
-r <repo>, --repo <repo> image repo
repo is required argument so we can make
it positional and can make it like docker:
docker pull cirros
zun pull cirros
Closes-Bug: #1658674
Change-Id: Iabbedc3e52a5c3f4db93eff052029d0fa2afff0e