Check if cinder is enabled before doing anything lvm2 related

On some system Cinder is not enabled so we can't assume LVM is installed. So
do not try to `sed` /etc/lvm/lvm.conf or clean LVM VG if cinder is not enabled

Change-Id: I09b1a7bee0785e5e1bb7dc96158a654bd3f15c83
diff --git a/unstack.sh b/unstack.sh
index f0da971..10e5958 100755
--- a/unstack.sh
+++ b/unstack.sh
@@ -187,5 +187,10 @@
 fi
 
 # BUG: maybe it doesn't exist? We should isolate this further down.
-clean_lvm_volume_group $DEFAULT_VOLUME_GROUP_NAME || /bin/true
-clean_lvm_filter
+# NOTE: Cinder automatically installs the lvm2 package, independently of the
+# enabled backends. So if Cinder is enabled, we are sure lvm (lvremove,
+# /etc/lvm/lvm.conf, etc.) is here.
+if is_service_enabled cinder; then
+    clean_lvm_volume_group $DEFAULT_VOLUME_GROUP_NAME || /bin/true
+    clean_lvm_filter
+fi