Basic versioning for images

The idea here is to have a version associated with an image.
When the image is created, the image gets version 1. Every time an update operation is done on the image, the version gets incremented.

Example of workflow:
glance image-create --name ubuntu
glance image-show
ubuntu version 1
glance image-update --name ubuntu-12.04-lts
glance image-show
ubuntu-12.04-lts version 2

At this point the version is just a way to differentiate an image over a period of time and gives the ability to see if an image has changed a lot in the past.

This blueprint should be seen as a starting point:
With this blueprint implemented, we can think at storing the previous state of the metadata and/or previous state of the bits in order to be able to rollback to the previous version of an image.