Changed Optional to Positional arg in zun pull

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

zunclient/v1/images_shell.py

@@ -18,10 +18,9 @@ def _show_image(image):
     utils.print_dict(image._info)
 
 
-@utils.arg('-r', '--repo',
-           required=True,
+@utils.arg('repo',
            metavar='<repo>',
-           help='image repo')
+           help='Image repository')
 def do_pull(cs, args):
     """Pull an image."""
     opts = {}