Allow to override cinder lvm device from cluster model

Add 'cinder_lvm_default_device' parameter with String type
to make possible to override it from cluster model.

Change-Id: I2deb7b642ba964a3e4b3d21a6f949c227f5bac4e
Related-Bug: https://mirantis.jira.com/browse/PROD-24014
diff --git a/cinder/volume/backend/lvm.yml b/cinder/volume/backend/lvm.yml
index 768fb6b..6559e46 100644
--- a/cinder/volume/backend/lvm.yml
+++ b/cinder/volume/backend/lvm.yml
@@ -1,6 +1,8 @@
 parameters:
   _param:
-    cinder_lvm_devices: [ "/dev/loop0" ]
+    cinder_lvm_default_device:  "/dev/loop0"
+    cinder_lvm_devices:
+    - ${_param:cinder_lvm_default_device}
   linux:
     storage:
       lvm: