Add VolumeMapping object for use in EC2

The EC2 code has a table mapping volume 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: I80e5d9c8e33c1f0c5ef8a9f7c54935c8c329ed25
Merge "Add VolumeMapping object for use in EC2"