Clean up session in db.sqla.api.get_ec2 methods

Remove session parameter from public methods

  get_ec2_volume_id_by_uuid
  get_volume_uuid_by_ec2_id
  get_ec2_snapshot_id_by_uuid
  get_snapshot_uuid_by_ec2_id
  get_ec2_instance_id_by_uuid
  get_instance_uuid_by_ec2_id

Because they are never called with session parameter
we could just remove it.

blueprint db-session-cleanup

Change-Id: Iecced3cf144de4f58a1169ed2d8f6940e229bedb
Merge "Clean up session in db.sqla.api.get_ec2 methods"