Add host parameter for backends

When cinder-volume & cinder-scheduler are shutdown on ctl-0x, we can't
add/remove volumes which are linked to specific service.
diff --git a/cinder/files/backend/_lvm.conf b/cinder/files/backend/_lvm.conf
index 04a5bd8..b71fa23 100644
--- a/cinder/files/backend/_lvm.conf
+++ b/cinder/files/backend/_lvm.conf
@@ -1,5 +1,6 @@
 
 [{{ backend_name }}]
+host={{ backend.get('host', grains.host) }}
 volume_driver = cinder.volume.drivers.lvm.LVMVolumeDriver
 volume_backend_name={{ backend_name }}
 lvm_type = default