Add EC2SnapshotMapping object

Create a nova object for the mappings between snapshot UUIDs and the
corresponding EC2 ID.

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

Change-Id: Ie29db77e404faff6ab9f12877094ea042e3c9924
Use EC2SnapshotMapping for creating mappings

Use the EC2SnapshotMapping nova object to create the mappings instead
of using the db api directly.

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

Change-Id: If280b1061185faf71c632a7bc4a8530346c18c97
Get EC2 snapshot mappings with nova object

Use the EC2SnapshotMapping nova object for getting the mapping instead
of directly using the db API.

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

Change-Id: I258ffe0a3fe3b0d7b2524bb64f1137127731bdd5
Remove unused db api methods

Remove methods that are no longer used from the db API.  New methods
were created that replaced these when the EC2SnapshotMapping object
was created in earlier commits.

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

Change-Id: I5431dd34d6a3fcea687b9d8f192a822e83ed9402
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 "Add EC2SnapshotMapping object"
Merge "Use EC2SnapshotMapping for creating mappings"
Merge "Get EC2 snapshot mappings with nova object"
Merge "Remove unused db api methods"
Merge "ec2: Add S3ImageMapping object"