ec2: Convert to use EC2InstanceMapping object

Update the EC2 code to make use of the EC2InstanceMapping object instead
of using the db API directly for managing instance UUID to EC2 ID
mappings.

Change-Id: I66d60a29f0d909edd381e8492a4754bdd0eb8e52
Merge "ec2: Convert to use EC2InstanceMapping object"