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
Merge "Use EC2SnapshotMapping for creating mappings"