EUCA-8651: VM certificate generation by EUARE
 * For elb/imaging/db, new properties "*vm_expiration_days" enforces the valid period of VM certificate issued to the VMs
 * EUARE prevents downloading server certificate if the VM certificate is invalid
 * For each service, admins are supposed to cleanup/re-start VMs after the expirion period; or they set expiration_days to larger value