nuke: make half-hearted attempt to sync before reboot

We don't want to block on sync for fear of a hung kernel
mount.  However, we can give it a try and wait a few seconds
to get what we can.

This fixes a problem where our recent modifications to the
sudoers file are lost, with a 0 byte file left in its place,
because the task fails and we do a reboot -f -n.

Signed-off-by: Sage Weil <sage@inktank.com>