The idea is to give us hooks for grabbing experimenters' tarballs (and
RPMs) from locations other than files on ops. Mainly, to remove
another dependance on users having shells on ops.

tarfiles_setup supports fetching files from http and ftp URLs right
now, through wget. It places them into the experiment directory, so
that they'll go away when the experiment is terminated, and the rest
of the chain (ie. downloading to clients and os_setup's checks)
remains unchaged.  It is now tarfiles_setup's job to copy tarballs and
RPMs from the virt_nodes table to the nodes table for allocated nodes.
This way, it can translate URLs into the local filenames it
constructs. It get invoked from tbswap.

Does the actual fetching over on ops, running as the user, with
fetchtar.proxy.

Should be idempotent, so we should be able to give the user a button
to run webtarfiles_setup (none exists yet) yet to 'freshen' their
tarballs. (We'd also have to somehow ...