Allow overriding RBD disk cache modes

Keep the default network=writeback setting, but allow it
to come from hiera overrides as well.

Conflicts:
manifests/profile/base/nova/compute_libvirt_shared.pp

Change-Id: I47aaf4be463684f5b67820b8ebb5db44ba960b44
Signed-off-by: Bogdan Dobrelya <bdobreli@redhat.com>
(cherry picked from commit f12a143f52864750fc90bb66c0e7eff0bed321c2)

manifests/profile/base/nova/compute_libvirt_shared.pp

@@ -38,8 +38,9 @@ class tripleo::profile::base::nova::compute_libvirt_shared (
 ) {
   if $step >= 4 {
     # Ceph + Libvirt
-    $rbd_ephemeral_storage = hiera('nova::compute::rbd::ephemeral_storage', false)
+    $rbd_ephemeral_storage  = hiera('nova::compute::rbd::ephemeral_storage', false)
     $rbd_persistent_storage = hiera('rbd_persistent_storage', false)
+    $rbd_disk_cachemodes    = hiera('rbd_disk_cachemodes', ['network=writeback'])
     if $rbd_ephemeral_storage or $rbd_persistent_storage {
       include nova::compute::rbd
       exec{ "exec-setfacl-${nova_rbd_client_name}-nova":
@@ -56,8 +57,8 @@ class tripleo::profile::base::nova::compute_libvirt_shared (
 
     if $rbd_ephemeral_storage {
       class { 'nova::compute::libvirt':
-        libvirt_disk_cachemodes => ['network=writeback'],
-        libvirt_hw_disk_discard => 'unmap',
+        disk_cachemodes => $rbd_disk_cachemodes,
+        hw_disk_discard => 'unmap',
       }
     } else {
       include nova::compute::libvirt

releasenotes/notes/rbd-disk-cache-modes-b6b75fa2e52b8915.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    New hiera setting `rbd_disk_cachemodes` allows to override
+    the disk cache modes for RBD. Defaults to ['network=writeback'].