Add EC2InstanceMapping object for use in EC2

The EC2 code has a table mapping instance UUIDs to EC2 IDs.  This patch
creates the Nova object for this so that we can remove direct use of
these db.api calls from the EC2 code.

Change-Id: Ib55426de29c1290ff9c7bcb999fb3b3124521750
Merge "Add EC2InstanceMapping object for use in EC2"