ceph-volume util.disk fix issue when capturing partition information

ceph-volume util.disk fix issue when capturing partition information

Before, the `if` condition meant that it would only work when the output
was '1', which is incorrect as that would only happen if a partition was
the first one, ignoring any other partition. The contents of that file
is the partition number, not a boolean to tell if it is a partition or
not.

It now includes the `holders` file contents which is needed for
dm-mapper work

Signed-off-by: Alfredo Deza <adeza@redhat.com>
(cherry picked from commit 28cb13a970909dc8efe508aee18f47e0fb3fdde3)