Add new ingressClass support to ingress provider

* add new ingressClass

* add doc

* lint

* adjust behavior to look for a class with a specific controller

* remove looking strange test ingressclass

* return nil rather than en empty object

* change documentation

* apply @kevinpollet suggestion

* change order of processIngress to be correct and adjust tests

* review: clean.

* review: clean.

* Fix for review

Co-authored-by: Manuel Zapf <manuel@containo.us>
Co-authored-by: Fernandez Ludovic <ludovic@containo.us>
Co-authored-by: Michael <michael.matur@gmail.com>

pkg/provider/kubernetes/ingress/fixtures/v18-Ingress-with-ingressClass_endpoint.yml

@@ -0,0 +1,11 @@
+kind: Endpoints
+apiVersion: v1
+metadata:
+  name: service1
+  namespace: testing
+
+subsets:
+- addresses:
+  - ip: 10.10.0.1
+  ports:
+  - port: 8080

pkg/provider/kubernetes/ingress/fixtures/v18-Ingress-with-ingressClass_ingress.yml

@@ -0,0 +1,14 @@
+kind: Ingress
+apiVersion: networking.k8s.io/v1beta1
+metadata:
+  name: ""
+  namespace: testing
+spec:
+  ingressClassName: traefik-lb
+  rules:
+  - http:
+      paths:
+      - path: /bar
+        backend:
+          serviceName: service1
+          servicePort: 80

pkg/provider/kubernetes/ingress/fixtures/v18-Ingress-with-ingressClass_ingressclass.yml

@@ -0,0 +1,6 @@
+apiVersion: networking.k8s.io/v1beta1
+kind: IngressClass
+metadata:
+  name: traefik-lb
+spec:
+  controller: traefik.io/ingress-controller

pkg/provider/kubernetes/ingress/fixtures/v18-Ingress-with-ingressClass_service.yml

@@ -0,0 +1,10 @@
+kind: Service
+apiVersion: v1
+metadata:
+  name: service1
+  namespace: testing
+
+spec:
+  ports:
+  - port: 80
+  clusterIp: 10.0.0.1

pkg/provider/kubernetes/ingress/fixtures/v18-Ingress-with-missing-ingressClass_endpoint.yml

@@ -0,0 +1,11 @@
+kind: Endpoints
+apiVersion: v1
+metadata:
+  name: service1
+  namespace: testing
+
+subsets:
+- addresses:
+  - ip: 10.10.0.1
+  ports:
+  - port: 8080

pkg/provider/kubernetes/ingress/fixtures/v18-Ingress-with-missing-ingressClass_ingress.yml

@@ -0,0 +1,14 @@
+kind: Ingress
+apiVersion: networking.k8s.io/v1beta1
+metadata:
+  name: ""
+  namespace: testing
+spec:
+  ingressClassName: traefik-lb
+  rules:
+  - http:
+      paths:
+      - path: /bar
+        backend:
+          serviceName: service1
+          servicePort: 80

pkg/provider/kubernetes/ingress/fixtures/v18-Ingress-with-missing-ingressClass_service.yml

@@ -0,0 +1,10 @@
+kind: Service
+apiVersion: v1
+metadata:
+  name: service1
+  namespace: testing
+
+spec:
+  ports:
+  - port: 80
+  clusterIp: 10.0.0.1