Minor fix for disk_image tags; do not include the tag if the OS is the type default. Let the other side figure it out.
Minor fix for disk_image tags; do not include the tag if the OS is the
type default. Let the other side figure it out.

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome