ceph-disk-prepare, debian/control: Support external journals.

ceph-disk-prepare, debian/control: Support external journals.

Previously, ceph-disk-* would only let you use a journal that was a
file inside the OSD data directory. With this, you can do:

  ceph-disk-prepare /dev/sdb /dev/sdb

to put the journal as a second partition on the same disk as the OSD
data (might save some file system overhead), or, more interestingly:

  ceph-disk-prepare /dev/sdb /dev/sdc

which makes it create a new partition on /dev/sdc to use as the
journal. Size of the partition is decided by $osd_journal_size.
/dev/sdc must be a GPT-format disk. Multiple OSDs may share the same
journal disk (using separate partitions); this way, a single fast SSD
can serve as journal for multiple spinning disks.

The second use case currently requires parted, so a Recommends: for
parted has been added to Debian packaging.

Closes: #3078
Closes: #3079
Signed-off-by: Tommi Virtanen <tv@inktank.com>