Fix ansible disk wipeoff skip

Fix ansible disk wipeoff skip

Previous commit b2e1386 switched from using a shell `-b` test to
using the ansible `stat` module for block device checks.
However, Ansible `stat` does not follow symlinks by default,
leading to some block devices being skipped, which left old
metadata in place, triggering an OSD activation failure as reported
in [1].

[1]
https://github.com/ceph/ceph-ansible/issues/2945#issuecomment-411720133

Fixes: b2e13865c2b899ce48148879da20323bd9bc988c

Reported-by: Krisztian Lengyel <krisztian.lengyel@nokia.com>
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>
Change-Id: Ie0f00cea2f9ff70a123b007a54e18122846456f3