Use ofagent-go as default, still support ofagent-py (and split the images name/tag)

Change-Id: If7f1b7b5daa587fd27241f79682095518f0a7f0f
diff --git a/voltha/Chart.yaml b/voltha/Chart.yaml
index 9111aab..d9dc664 100644
--- a/voltha/Chart.yaml
+++ b/voltha/Chart.yaml
@@ -17,7 +17,7 @@
 description: A Helm chart for Voltha based on K8S resources in Voltha project
 icon: https://guide.opencord.org/logos/cord.svg
 
-version: 2.2.7
+version: 2.2.8
 
 # app version applies to multiple components with potentially different
 # container image versions