Shared file support for tools/burnin

Make burnin work with -t sharedfile and update burnin's help.

Shared block storage support is *not* included, as currently there is no way to
provision shared block storage from within Ganeti.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>