Adds support for Multus (multiple interfaces) CNI plugin (#3166)

* Adds support for Multus (multiple interfaces) CNI plugin

Multus is a latin word for "Multi". As the name suggests, it acts as a
Multi plugin in Kubernetes and provides multiple network interface
support in a pod. Multus uses the concept of invoking delegates by
grouping multiple plugins into delegates and invoking them in the
sequential order of the CNI configuration file provided in json format.

* Change CNI version (0.1.0->0.3.1) of Contiv to be compatible with Multus

roles/network_plugin/multus/files/multus-crd.yml

@@ -0,0 +1,22 @@
+---
+kind: CustomResourceDefinition
+apiVersion: apiextensions.k8s.io/v1beta1
+metadata:
+  name: network-attachment-definitions.k8s.cni.cncf.io
+spec:
+  group: k8s.cni.cncf.io
+  version: v1
+  scope: Namespaced
+  names:
+    plural: network-attachment-definitions
+    singular: network-attachment-definition
+    kind: NetworkAttachmentDefinition
+    shortNames:
+    - net-attach-def
+  validation:
+    openAPIV3Schema:
+      properties:
+        spec:
+          properties:
+            config:
+              type: string