Enable client V2 to download images

Added the CLI option image-download to download an image via API V2.
Added utility function to save an image.
Added common iterator to validate the checksum.

Related to bp glance-client-v2

Change-Id: I0247f5a3462142dc5e9f3dc16cbe00c8e3d42f42

glanceclient/v2/shell.py

@@ -49,3 +49,14 @@ def do_explain(gc, args):
         formatters = {'Attribute': lambda m: m.name}
         columns = ['Attribute', 'Description']
         utils.print_list(schema.properties, columns, formatters)
+
+
+@utils.arg('--file', metavar='<FILE>',
+           help='Local file to save downloaded image data to. '
+                'If this is not specified the image data will be '
+                'written to stdout.')
+@utils.arg('id', metavar='<IMAGE_ID>', help='ID of image to download.')
+def do_image_download(gc, args):
+    """Download a specific image."""
+    body = gc.images.data(args.id)
+    utils.save_image(body, args.file)