Several changes:
Several changes:

1. Do not pass -u option (update mode) to the mapper call in GetTicket, if
   not actually an update. This will now cause the mapper to pass the -F
   option to assign, so that fixnode (by the user rspec) will not override
   normal constraint checking.

2. MAINSITE: Add d820-explicit desire so we can actually use the shared
   node pool.

3. Add checks for out of memory errors from assign, and pass that back for
   the caller.

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