ceph-disk: more robust parted output parser

ceph-disk: more robust parted output parser

In some cases, depending on the implementation or the operating system,

   parted --machine -- /dev/sdh print

may contain empty lines. The current parsing code is fragile and highly
depends on output details. Replace it with code that basically does the
same sanity checks (output not empty, existence of units, existence of
the dev entry) but handles the entire output instead of checking line by
line.

http://tracker.ceph.com/issues/10983 Fixes: #10983

Signed-off-by: Loic Dachary <ldachary@redhat.com>