added tutorial

docs/tutorials/openshift.md

+# Configuring ExternalDNS to use the OpenShift Route Source
+This tutorial describes how to configure ExternalDNS to use the OpenShift Route source.
+It is meant to supplement the other provider-specific setup tutorials.
+
+### Prepare ROUTER_CANONICAL_HOSTNAME in default/router deployment
+Read and go through [Finding the Host Name of the Router](https://docs.openshift.com/container-platform/3.11/install_config/router/default_haproxy_router.html#finding-router-hostname).
+If no ROUTER_CANONICAL_HOSTNAME is set, you must annotate each route with external-dns.alpha.kubernetes.io/target!
+
+### Manifest (for clusters without RBAC enabled)
+```yaml
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: external-dns
+spec:
+  strategy:
+    type: Recreate
+  selector:
+    matchLabels:
+      app: external-dns
+  template:
+    metadata:
+      labels:
+        app: external-dns
+    spec:
+      containers:
+      - name: external-dns
+        image: registry.opensource.zalan.do/teapot/external-dns:latest
+        args:
+        - --source=openshift-route
+        - --domain-filter=external-dns-test.my-org.com # will make ExternalDNS see only the hosted zones matching provided domain, omit to process all available hosted zones
+        - --provider=aws
+        - --policy=upsert-only # would prevent ExternalDNS from deleting any records, omit to enable full synchronization
+        - --aws-zone-type=public # only look at public hosted zones (valid values are public, private or no value for both)
+        - --registry=txt
+        - --txt-owner-id=my-identifier
+```
+
+### Manifest (for clusters with RBAC enabled)
+```yaml
+apiVersion: v1
+kind: ServiceAccount
+metadata:
+  name: external-dns
+---
+apiVersion: rbac.authorization.k8s.io/v1beta1
+kind: ClusterRole
+metadata:
+  name: external-dns
+rules:
+- apiGroups: [""]
+  resources: ["services","endpoints","pods"]
+  verbs: ["get","watch","list"]
+- apiGroups: ["extensions"] 
+  resources: ["ingresses"] 
+  verbs: ["get","watch","list"]
+- apiGroups: [""]
+  resources: ["nodes"]
+  verbs: ["list"]
+- apiGroups: ["route.openshift.io"]
+  resources: ["routes"]
+  verbs: ["get","watch","list"]
+---
+apiVersion: rbac.authorization.k8s.io/v1beta1
+kind: ClusterRoleBinding
+metadata:
+  name: external-dns-viewer
+roleRef:
+  apiGroup: rbac.authorization.k8s.io
+  kind: ClusterRole
+  name: external-dns
+subjects:
+- kind: ServiceAccount
+  name: external-dns
+  namespace: default
+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: external-dns
+spec:
+  strategy:
+    type: Recreate
+  selector:
+    matchLabels:
+      app: external-dns
+  template:
+    metadata:
+      labels:
+        app: external-dns
+    spec:
+      serviceAccountName: external-dns
+      containers:
+      - name: external-dns
+        image: registry.opensource.zalan.do/teapot/external-dns:latest
+        args:
+        - --source=openshift-route
+        - --domain-filter=external-dns-test.my-org.com # will make ExternalDNS see only the hosted zones matching provided domain, omit to process all available hosted zones
+        - --provider=aws
+        - --policy=upsert-only # would prevent ExternalDNS from deleting any records, omit to enable full synchronization
+        - --aws-zone-type=public # only look at public hosted zones (valid values are public, private or no value for both)
+        - --registry=txt
+        - --txt-owner-id=my-identifier
+```
+
+### Verify External DNS works (OpenShift Route example)
+The following instructions are based on the 
+[Hello Openshift](https://github.com/openshift/origin/tree/master/examples/hello-openshift).
+
+#### Install a sample service and expose it
+```bash
+$ oc apply -f - <<EOF
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  labels:
+    app: hello-openshift
+  name: hello-openshift
+spec:
+  replicas: 1
+  selector:
+    matchLabels:
+      app: hello-openshift
+  template:
+    metadata:
+      labels:
+        app: hello-openshift
+    spec:
+      containers:
+      - image: openshift/hello-openshift
+        name: hello-openshift
+---
+apiVersion: v1
+kind: Service
+metadata:
+  labels:
+    app: hello-openshift
+  name: hello-openshift
+spec:
+  ports:
+  - port: 8080
+    protocol: TCP
+    targetPort: 8080
+  selector:
+    app: hello-openshift
+  sessionAffinity: None
+  type: ClusterIP
+---
+apiVersion: route.openshift.io/v1
+kind: Route
+metadata:
+  name: hello-openshift
+spec:
+  host: hello-openshift.example.com
+  to:
+    kind: Service
+    name: hello-openshift
+    weight: 100
+  wildcardPolicy: None
+EOF
+```
+
+#### Access the sample route using `curl`
+```bash
+$ curl -i http://hello-openshift.example.com
+HTTP/1.1 200 OK
+Date: Fri, 10 Apr 2020 09:36:41 GMT
+Content-Length: 17
+Content-Type: text/plain; charset=utf-8
+
+Hello OpenShift!
+```