nuke: ignore exceptions while issuing reboot command

I'm seeing failed tasks (and nuke) leak machines.  It looks like we are
getting an exception on the '... reboot -f -n' command when we should be
ignoring it and waiting for the machine to restart.

For example:
   http://qa-proxy.ceph.com/teuthology/sage-2013-12-08_19:25:06-rados:thrash-wip-tier-foo-basic-plana/136321/teuthology.log

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