ceph-disk-prepare: allow a prepare on a dir

If we get passed a dir, prepare it, and skip all the device nonsense.

FIXME: maybe should still create a journal?

Signed-off-by: Sage Weil <sage@inktank.com>
ceph-disk-active: fix --dir DIR behavior

Various typos.  Safely update the symlink.

Signed-off-by: Sage Weil <sage@inktank.com>
ceph-disk-prepare: handle any combination of file/partition/blkdev for data/journal

Signed-off-by: Sage Weil <sage@inktank.com>
ceph-disk-activate: remove in-the-way symlinks in /var/lib/ceph/osd

Signed-off-by: Sage Weil <sage@inktank.com>
ceph-disk-activate: be marginally more helpful

Either --dir or --mount is required.

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