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/ro-core.yaml b/voltha/templates/ro-core.yaml
index 7553c28..0879cc0 100644
--- a/voltha/templates/ro-core.yaml
+++ b/voltha/templates/ro-core.yaml
@@ -104,7 +104,7 @@
       serviceAccountName: {{ .Values.serviceaccount }}
       containers:
         - name: voltha
-          image: {{ tpl .Values.images.ro_core.registry . }}{{ tpl .Values.images.ro_core.repository . }}:{{ tpl ( tpl .Values.images.ro_core.tag . ) . }}
+          image: '{{ tpl .Values.images.ro_core.registry . }}{{ tpl .Values.images.ro_core.repository . }}:{{ tpl .Values.images.ro_core.tag . }}'
           imagePullPolicy: {{ tpl .Values.images.ro_core.pullPolicy . }}
           env:
             - name: NAMESPACE