[VOL-3091] configurable kafka topic

Make core-topic and adapter topics configurable in voltha-helm-charts
This is required to run multiple voltha stacks in a k8s deployment.

Also, read the kv_store_data_prefix via environment variable.
https://gerrit.opencord.org/#/c/19001/

Change-Id: I8fabda3ce70f1379f735796e6711f4282e57008a
diff --git a/voltha/values.yaml b/voltha/values.yaml
index 52b72a7..cb2b9e1 100644
--- a/voltha/values.yaml
+++ b/voltha/values.yaml
@@ -24,6 +24,13 @@
   image_pullPolicy: "Always"
   rw_core:
     timeout: "8000"
+  topics:
+    core_topic: "rwcore"
+    
+  # Define prefix path for etcd
+  # If you're running multiple voltha stacks the this value 
+  # should be unique across the stacks.
+  kv_store_data_prefix: "service/voltha"
 
 securityContext:
   enabled: true