EUCA-8133: download server certificate from Euare
  As part of ELB ssl termination, ELB servo needs to download the server certificate identified with its ARN.
  To ensure confidentiality, we encrypt the private key of the server cert using the instance key-pair
  that is delivered using the existing trust chain between clc-nc and VM's floppy (EUCA-8131).
  DownloadServerCertificate is a new euca-specific API in EUARE for consumption by ELB servo.