Upgrade helm to 2.15 for nested chart dependency

Change-Id: I1fdc719865e403b4b2d13f6951b430f726afac3b
diff --git a/comac/sample/kubespray-inventories/single/extra-vars.yaml b/comac/sample/kubespray-inventories/single/extra-vars.yaml
index c4549ab..b67911b 100644
--- a/comac/sample/kubespray-inventories/single/extra-vars.yaml
+++ b/comac/sample/kubespray-inventories/single/extra-vars.yaml
@@ -52,4 +52,4 @@
 
 helm_enabled: true
 helm_deployment_type: host
-helm_version: v2.14.2
+helm_version: v2.15.0