Update command descriptions

Several commands did not have descriptions or the descriptions
they had were insufficient. This adds mission descriptions
and fattens up those that were too lean.

Change-Id: I091ae70cdae5d3f72f273519d88873cb5392ba3b

glanceclient/v2/shell.py

@@ -20,7 +20,7 @@ from glanceclient import exc
 @utils.arg('--page-size', metavar='<SIZE>', default=None, type=int,
            help='Number of images to request in each paginated request.')
 def do_image_list(gc, args):
-    """List images."""
+    """List images you can access."""
     kwargs = {}
     if args.page_size is not None:
         kwargs['page_size'] = args.page_size