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/api-server.yaml b/voltha/templates/api-server.yaml
index c14c881..8a8f59f 100644
--- a/voltha/templates/api-server.yaml
+++ b/voltha/templates/api-server.yaml
@@ -74,7 +74,7 @@
       serviceAccountName: {{ .Values.serviceaccount }}
       containers:
       - name: arouter
-        image: {{ tpl .Values.images.afrouter.registry . }}{{ tpl .Values.images.afrouter.repository . }}:{{ tpl ( tpl .Values.images.afrouter.tag . ) . }}
+        image: '{{ tpl .Values.images.afrouter.registry . }}{{ tpl .Values.images.afrouter.repository . }}:{{ tpl .Values.images.afrouter.tag . }}'
         imagePullPolicy: {{ tpl .Values.images.afrouter.pullPolicy . }}
         volumeMounts:
         - name: config-volume
@@ -96,7 +96,7 @@
           value: {{ quote .Values.services.kafka.cluster.port }}
         - name: KAFKA_TOPIC
           value: {{ quote .Values.api_server.kafka_topic }}
-        image: {{ tpl .Values.images.afrouterd.registry . }}{{ tpl .Values.images.afrouterd.repository . }}:{{ tpl ( tpl .Values.images.afrouterd.tag . ) . }}
+        image: '{{ tpl .Values.images.afrouterd.registry . }}{{ tpl .Values.images.afrouterd.repository . }}:{{ tpl .Values.images.afrouterd.tag . }}'
         imagePullPolicy: {{ tpl .Values.images.afrouterd.pullPolicy . }}
         command: ["/app/arouterd"]
       restartPolicy: Always