ceph-disk: make initial journal files 0 bytes

The ceph-osd will resize journal files up and properly fallocate() them
so that the blocks are preallocated and (hopefully) contiguous.  We
don't need to do it here too, and getting fallocate() to work from
python is a pain in the butt.

Fixes: #5981
Signed-off-by: Sage Weil <sage@inktank.com>