[VOL-1866] Changed module dependency to v12.0.0 of k8s client-go and v1.15.4 of k8s api/apimachinery in sync with other voltha components

Had to use pseudo-version corresponding to v12.0.0 of k8s client-go
because golang proxy is no longer serving the modules not complying
to Semantic Import Versioning rules including client-go v12.0.0.
Refer to https://github.com/kubernetes/client-go/issues/631 and
https://github.com/golang/go/issues/33558

Change-Id: I2e558bab7f0702f230761319eb5392a7d0532ea3
diff --git a/vendor/k8s.io/api/rbac/v1/types.go b/vendor/k8s.io/api/rbac/v1/types.go
index 17163cb..7ba7d05 100644
--- a/vendor/k8s.io/api/rbac/v1/types.go
+++ b/vendor/k8s.io/api/rbac/v1/types.go
@@ -108,6 +108,7 @@
 	metav1.ObjectMeta `json:"metadata,omitempty" protobuf:"bytes,1,opt,name=metadata"`
 
 	// Rules holds all the PolicyRules for this Role
+	// +optional
 	Rules []PolicyRule `json:"rules" protobuf:"bytes,2,rep,name=rules"`
 }
 
@@ -170,6 +171,7 @@
 	metav1.ObjectMeta `json:"metadata,omitempty" protobuf:"bytes,1,opt,name=metadata"`
 
 	// Rules holds all the PolicyRules for this ClusterRole
+	// +optional
 	Rules []PolicyRule `json:"rules" protobuf:"bytes,2,rep,name=rules"`
 
 	// AggregationRule is an optional field that describes how to build the Rules for this ClusterRole.