Image-Volume cache

Introduce `image_volume_cache_enabled` parameter for ceph backend.

Optional parameter for enabling cinder image-volume cache.
By default set to false.

Docs: https://docs.openstack.org/cinder/latest/admin/blockstorage-image-volume-cache.html
diff --git a/tests/pillar/ceph_single.sls b/tests/pillar/ceph_single.sls
index bc15f19..761943b 100644
--- a/tests/pillar/ceph_single.sls
+++ b/tests/pillar/ceph_single.sls
@@ -11,6 +11,7 @@
         user: cinder
         secret_uuid: password
         client_cinder_key: password
+        image_volume_cache_enabled: true
     identity:
       engine: keystone
       host: 127.0.0.1