Add support for explicit type request feature/desire. When there is an explicit_request attribute in the node_type_attributes table for a type, and the topology explicitly requests that type, add a desire to the vtop file call $type-explicit:1.0. An equivalent feature added by ptopgen will result in those nodes not being allocated unless explicitly requested.
Add support for explicit type request feature/desire. When there is
an explicit_request attribute in the node_type_attributes table for a
type, and the topology explicitly requests that type, add a desire
to the vtop file call $type-explicit:1.0. An equivalent feature added
by ptopgen will result in those nodes not being allocated unless
explicitly requested.

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