ceph::osd : add tests for osd data on an existing partition

Specifying an existing partition as osd data is supported when the
partition is properly set with the Ceph type uuid (
4fbd7e29-9d25-41b8-afd0-062c0ceff05d ). Add a test that shows it is.

Change-Id: I186647e331157a5ea8df025c0074e38a25cb238a
Signed-off-by: Loic Dachary <loic@dachary.org>