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

tests/files/gce_centos7-multus-calico.yml

@@ -0,0 +1,12 @@
+# Instance settings
+cloud_image_family: centos-7
+cloud_region: us-central1-c
+cloud_machine_type: "n1-standard-1"
+mode: default
+
+# Deployment settings
+kube_network_plugin_multus: true
+kube_network_plugin: calico
+deploy_netchecker: true
+kubedns_min_replicas: 1
+cloud_provider: gce