[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/core/v1/annotation_key_constants.go b/vendor/k8s.io/api/core/v1/annotation_key_constants.go
index 2c72ec2..edc9b4d 100644
--- a/vendor/k8s.io/api/core/v1/annotation_key_constants.go
+++ b/vendor/k8s.io/api/core/v1/annotation_key_constants.go
@@ -97,4 +97,10 @@
 	// This annotation will be used to compute the in-cluster network programming latency SLI, see
 	// https://github.com/kubernetes/community/blob/master/sig-scalability/slos/network_programming_latency.md
 	EndpointsLastChangeTriggerTime = "endpoints.kubernetes.io/last-change-trigger-time"
+
+	// MigratedPluginsAnnotationKey is the annotation key, set for CSINode objects, that is a comma-separated
+	// list of in-tree plugins that will be serviced by the CSI backend on the Node represented by CSINode.
+	// This annotation is used by the Attach Detach Controller to determine whether to use the in-tree or
+	// CSI Backend for a volume plugin on a specific node.
+	MigratedPluginsAnnotationKey = "storage.alpha.kubernetes.io/migrated-plugins"
 )