ceph-disk-prepare: Allow specifying fs type to use.

ceph-disk-prepare: Allow specifying fs type to use.

Either use ceph.conf variable osd_fs_type or command line option
--fs-type=

Default is still ext4, as currently nothing guarantees xfsprogs
or btrfs-tools are installed.

Currently both btrfs and xfs seems to trigger a disk hotplug event at
mount time, thus triggering a useless and unwanted ceph-disk-activate
run. This will be worked around in a later commit.

Currently mkfs and mount options cannot be configured.

Bug: #2549
Signed-off-by: Tommi Virtanen <tv@inktank.com>