Enforce image_size_cap on v2 upload

image_size_cap should be checked and enforced on upload

Enforcement is in two places:
- on image metadata save
- during image save to backend store

Closes-Bug: 1315321
Change-Id: I45bfb360703617bc394e9e27fe17adf43b09c0e1
Co-Author: Manuel Desbonnet <manuel.desbonnet@hp.com>