Make sure the empty image file we create is group writable.
Make sure the empty image file we create is group writable.

create_image runs as the invoking user but the frisbee uploader may
run as a different user (the creator of the image). If the file is
not group writable, we will get a permission denied error in the
uploader.

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