nginx version bump & doc updates (#448)

Bump nginx from 1.13 to 1.15.

Remove nginx from list of components for which replicaCount>1
is not supported. Stateless component -- running multiple replicas
is expected to just work.
diff --git a/docs/configurationChoices.md b/docs/configurationChoices.md
index 38c65d5..4e522a5 100644
--- a/docs/configurationChoices.md
+++ b/docs/configurationChoices.md
@@ -42,7 +42,6 @@
 - couchdb
 - kafka
 - kafkaprovider
-- nginx
 - redis
 We are actively working on reducing this list and would welcome PRs to help.
 
diff --git a/helm/openwhisk/values.yaml b/helm/openwhisk/values.yaml
index e9bf7ef..ec629db 100644
--- a/helm/openwhisk/values.yaml
+++ b/helm/openwhisk/values.yaml
@@ -210,9 +210,8 @@
 # Nginx configurations
 nginx:
   imageName: "nginx"
-  imageTag: "1.13"
+  imageTag: "1.15"
   imagePullPolicy: "IfNotPresent"
-  # NOTE: setting replicaCount > 1 is not tested and may not work
   replicaCount: 1
   restartPolicy: "Always"
   httpPort: 80