ec2: Add S3ImageMapping object

Add a nova object for another ID-UUID mapping used in the EC2 API.
This object will allow the removal of the last bit of direct db api
usage by the EC2 API code.

Part of blueprint convert-ec2-api-to-use-nova-objects

Change-Id: Icf84f2d5a50ce4c78ee7f51db8e6a2b0276afbb2
Merge "ec2: Add S3ImageMapping object"