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

README.md

@@ -125,6 +125,7 @@ Supported Components
     -   [weave](https://github.com/weaveworks/weave) v2.4.1
     -   [kube-router](https://github.com/cloudnativelabs/kube-router) v0.2.1
 -   Application
+    -   [multus](https://github.com/intel/multus-cni) v3.1
     -   [cephfs-provisioner](https://github.com/kubernetes-incubator/external-storage) v2.1.0-k8s1.11
     -   [cert-manager](https://github.com/jetstack/cert-manager) v0.5.0
     -   [coredns](https://github.com/coredns/coredns) v1.2.5
@@ -176,6 +177,8 @@ You can choose between 6 network plugins. (default: `calico`, except Vagrant use
     iptables for network policies, and BGP for ods L3 networking (with optionally BGP peering with out-of-cluster BGP peers).
     It can also optionally advertise routes to Kubernetes cluster Pods CIDRs, ClusterIPs, ExternalIPs and LoadBalancerIPs.
 
+-   [multus](docs/multus.md): Multus is a meta CNI plugin that provides multiple network interface support to pods. For each interface Multus delegates CNI calls to secondary CNI plugins such as Calico, macvlan, etc.
+
 The choice is defined with the variable `kube_network_plugin`. There is also an
 option to leverage built-in cloud provider networking instead.
 See also [Network checker](docs/netcheck.md).