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
ec2: create volume mapping using nova object

Update the EC2 code that creates a mapping in the db between the
volume UUID and an EC2 ID to use the nova object for this.

Change-Id: I2b02c9f639519a7835ffcf27be704b4810334e6d
ec2utils: Use VolumeMapping object

Update ec2utils to use the VolumeMapping object instead of direct db
API calls for getting the ec2 ID to UUID mappings.

Change-Id: Id653fc16fffc1c416379087228a0b053b8868cac
Merge "Add VolumeMapping object for use in EC2"
Merge "ec2: create volume mapping using nova object"
Merge "ec2utils: Use VolumeMapping object"