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-adapter-openonu/templates/adapters-openonu.yaml b/voltha-adapter-openonu/templates/adapters-openonu.yaml
index 98c4b3e..475f018 100644
--- a/voltha-adapter-openonu/templates/adapters-openonu.yaml
+++ b/voltha-adapter-openonu/templates/adapters-openonu.yaml
@@ -60,7 +60,7 @@
     spec:
       containers:
         - name: adapter-open-onu
-          image: {{ tpl .Values.images.adapter_open_onu.registry . }}{{ tpl .Values.images.adapter_open_onu.repository . }}:{{ tpl ( tpl .Values.images.adapter_open_onu.tag . ) . }}
+          image: '{{ tpl .Values.images.adapter_open_onu.registry . }}{{ tpl .Values.images.adapter_open_onu.repository . }}:{{ tpl ( tpl .Values.images.adapter_open_onu.tag . ) . }}'
           imagePullPolicy: {{ tpl .Values.images.adapter_open_onu.pullPolicy . }}
           args:
             - "/voltha/adapters/brcm_openomci_onu/main.py"