ceph-disk: blacklist /dev/fd0

blkid -s TYPE /dev/fd0 has been verified to hang forever on a
H8DMR-82 supermicro motherboard running

3.8.0-33-generic #48~precise1-Ubuntu SMP Thu Oct 24 16:28:06 UTC 2013
x86_64

It is unlikely that ceph will ever be used on floppy disks, they
can be blacklisted.

http://tracker.ceph.com/issues/6827 fixes: #6827

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #860 from dachary/wip-6827

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>