Decouple EC2 API from using instance id

Continuation of 0dc32690fe158e4cb11c2c9bcc65acaf73b94a7a

Some code in ec2 api use Instance.id rather than InstanceIdMapping.id
directly while converting ec2 id to an instance object. This is very
dangerous if Instance.id is not equal to InstanceIdMapping.id for the
same instance uuid.

To avoid the id mapping confusion, this patch:

 * uses instance_id_mappings.id instead of instances.id
 * uses instance_uuid in ec2utils.id_to_ec2_inst_id
 * removes ec2utils.ec2_instance_id_to_uuid

Fixes bug 1145490

Change-Id: I22638f667c18eefe542b03e31f1a3aa2ce782db7
(cherry picked from commit 79cc2a21b645138f70c6ca4b54053f69c0dde81d)
Merge "Decouple EC2 API from using instance id" into stable/folsom