ceph-disk: Enable creating multiple osds per dev

This is useful for demos and testing.  Enables
creation of lots of OSDs on a single block device
by simply running ceph-disk prepare more than once,
with a --data-size argument set.

Signed-off-by: John Spray <john.spray@inktank.com>
Merge pull request #1859 from ceph/wip-multipartition

ceph-disk: Enable creating multiple osds per dev

Reviewed-by: Dan Mick <dan.mick@inktank.com>