Add lookup by urn (where the urn holds the internal index number). Also add a GetManifest method to avoid code duplication.
Add lookup by urn (where the urn holds the internal index number).
Also add a GetManifest method to avoid code duplication.

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