Assign local variable in api.v2.image_data

If local variable 'image' will be not initialized we get UnboundLocalError
while handle following exceptions:
exception.StorageFull
exception.StorageQuotaFull
exception.ImageSizeLimitExceeded
exception.StorageWriteDenied
webob.exc.HTTPError
ValueError

This is because not initialized variable 'image' used as argument of
method _restore. That is why we should initialize it.

Closes-Bug: #1310952

Change-Id: I3bf73ca272aad479416067495006476934a01dd9
Merge "Assign local variable in api.v2.image_data"