Fixes HTTP 500 when updating image with locations for V2

Based on current implement, user will run into 500 error during
updating locations for a queued image. The root cause is we're
trying to calculate the overall size with current image size and
the number of locations. This fix will get the correct image size
from backend store if current image size is not available.

Fixes bug 1257507

Change-Id: Ia4ac73e36211d475752ee9f4995fc3768fb2981a
Merge "Fixes HTTP 500 when updating image with locations for V2"