Update script for frisbee MFS.
Update script for frisbee MFS.

Takes care of downloading the update tarfile, mounting the MFS and
installing the tarfile.

Right now, this is pretty specific to updating the frisbee MFS.
It should be generalized to the other MFSes and, to the extent it makes
sense, reconciled with the existing update mechanism.

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