ceph_key: `lookup_ceph_initial_entities` shouldn't fail on update

ceph_key: `lookup_ceph_initial_entities` shouldn't fail on update

As of nautilus, the initial keyrings list has changed, it means when
upgrading from Luminous or Mimic, it is expected there's a mismatch
between what is found on the cluster and the expected initial keyring
list hardcoded in ceph_key module. We shouldn't fail when upgrading to
nautilus.

str_to_bool() took from ceph-volume.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
Co-Authored-by: Alfredo Deza <adeza@redhat.com>