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>