Bump API version to 2.3

Some changes have landed in Glance's tree during Kilo that require a
minor bump to the API. The following change is probably the one that had
the biggest impact on the current v2.2 API.

- Allow null fields to be returned: Ieaddd8a686cf7361f18cb1ee83b7887cdca22bd6

ApiImpact
DocImpact

Change-Id: I214ae6466ac85876cc9589069913258e80db29b5
Merge "Bump API version to 2.3"