Allow updating empty created v2 images from v1

When an empty image is created through python-glanceclient using v2 APIs
but updated using v1 APIs, python-glanceclient crashes with a "int()
argument must be a string or a number, not 'NoneType'" message.

This is because v1.images.ImageManager._format_image_meta_for_user
expects the 'size' (and 'min_ram' and 'min_disk') field of an image to
be convertible to 'int' but the server-side v2 APIs set it to 'None' for
an empty image. Therefore the conversion to int fails with the message
above.

This fix modifies _format_image_meta_for_user to check whether or not a
field can be converted to 'int' and, if not, returns 0 (zero) as the
value instead.

Tests have also been added.

Change-Id: I86680bc06c8ce3ee492efeb3f32071da4f293bcd
Closes-bug: #1258160

glanceclient/v1/images.py

@@ -98,7 +98,7 @@ class ImageManager(base.Manager):
         for key in ['size', 'min_ram', 'min_disk']:
             if key in meta:
                 try:
-                    meta[key] = int(meta[key])
+                    meta[key] = int(meta[key]) if meta[key] else 0
                 except ValueError:
                     pass
         return meta