Add default features flag to ceph.conf generator

Coupled with the removal of explicit --image-features flags to the RBD
command in PVC itself, this ensures that only the two features supported
on kernel 4.19 are enabled by default.

roles/pvc/tasks/ceph/bootstrap.yml

@@ -72,6 +72,7 @@
     - "osd pool default pgp num = 256"
     - "osd crush chooseleaf type = 1"
     - "bluestore cache autotune = true"
+    - "rbd default features = 5"
 
 - name: add memory limit if requested
   lineinfile: