Add new db api get functions for ec2_snapshot

This patch adds two new getters for EC2 snapshot mappings to the db
API.  The existing getters don't return the full db model.  Instead,
they only return an ID or UUID.  The full db model is needed when this
gets converted over to a nova object.

Related to blueprint convert-ec2-api-to-use-nova-objects

Change-Id: I8e2ed5086a7768f373d35c2f34386f2d30d11e07
Merge "Add new db api get functions for ec2_snapshot"