VOL-2110 - update how default image tags are managed

- Sneak in a work around for helm 2.15

Change-Id: Iabe4fa98b5b4a574061f1fac7a876b3f1f05c398
diff --git a/voltha/templates/ofagent.yaml b/voltha/templates/ofagent.yaml
index ca8e5d5..0e9dc49 100644
--- a/voltha/templates/ofagent.yaml
+++ b/voltha/templates/ofagent.yaml
@@ -60,7 +60,7 @@
       serviceAccountName: {{ .Values.serviceaccount }}
       containers:
       - name: ofagent
-        image: {{ tpl .Values.images.ofagent.registry . }}{{ tpl .Values.images.ofagent.repository . }}:{{ tpl ( tpl .Values.images.ofagent.tag . ) . }}
+        image: '{{ tpl .Values.images.ofagent.registry . }}{{ tpl .Values.images.ofagent.repository . }}:{{ tpl .Values.images.ofagent.tag . }}'
         imagePullPolicy: {{ tpl .Values.images.ofagent.pullPolicy . }}
         env:
         - name: CONTROLLER_SERVICE