When using external ceph without nova integration, kolla-ansible fails
because 'nova_cephx_raw_key' is undefined.
This patch fixes the issue by applying the 'default' filter to
prevent failure on undefined.
The change in behavior was introduced by [1].
[1] https://review.opendev.org/689753
commit 44709f413246a05b891f250e88c43226e1f1b0c9
Change-Id: I2fdca1a6a78e78623733a387a2d8c7e29d449083
Closes-Bug: #1853862
Co-Authored-By: Mark Goddard <mark@stackhpc.com>
6ff6337230