library: import ca_test_common in test_ceph_key
library: import ca_test_common in test_ceph_key

since we added `ca_test_common.py` let's use it in `test_ceph_key.py`

Signed-off-by: Guillaume Abrioux <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>

ceph/ceph-ansible,ceph/ceph-ansible