The MVP of image import functionality will be introduced as an
EXPERIMENTAL 2.6 version of the Images API. The current version
will remain at 2.5.
This patch:
* Updates the version response
* Updates version negotiation code
* Updates unit and functional tests
* Updates the api-ref
* Adds a releasenote
Change-Id: Ied573897157e05f7b0438baeb1553326fb06613a