Fix OSD weight and device size

Because of the following bug: http://tracker.ceph.com/issues/8551
If we use a disk file size of 1GB the OSD weight calculation ends up
being 0. Thus no data will be stored on any OSD.
Increasing the disk file size up to 11GB (we save some filesystem
overhead since the calculation is based on df thus 11GB is safer than
10GB). Because we significantly increased the size of the disk files we
now only create 2 devices per OSD host.

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>