The function add calculation size_gb need improve
The function add calculation size_gb need improve

The line:size_gb = int((image_size + units.Gi - 1) / units.Gi).
Its intent is to get ceiling of size_gb. we can use python math
module math.ceil() function. This can improve code readability
and more in line with the python style. This patch improves it.

Change-Id: I0171d78a55810c35d1b6dbb8632c3d0594f287c1
Closes-Bug: #1592376

openstack/glance_store,openstack/glance_store