Allow None values to be returned from the API

Currently, Glance's API v2 doesn't return fields whose value is None.
This, unfortunately, is wrong for a client perspective since it would
create inconsistencies between calls and images due to the lack of
fields in the response.

The API should guarantee consistency in its replies and ensure all
fields have a value, even if it's None.

NOTE: This work is part of the migration to v2. It fixes inconsistencies
in the API and improves the interaction between the client library and
Glance.

NOTE2: A follow-up patch will bump the minor API version, wait for it.

ApiImpact
DocImpact
Closes-bug: #1398314

Change-Id: Ieaddd8a686cf7361f18cb1ee83b7887cdca22bd6
Merge "Allow None values to be returned from the API"