Ports image_size extension to v3 API

This merges the functionality from the v2 image_size functionality into
the images extension (which is no longer core in v3)

Partially implements blueprint nova-v3-api

Change-Id: I577907e671d18712893afcb5477fbd2aa6031956
Merge "Ports image_size extension to v3 API"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp