[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/client-go/util/cert/OWNERS b/vendor/k8s.io/client-go/util/cert/OWNERS
index 470b7a1..3cf0364 100644
--- a/vendor/k8s.io/client-go/util/cert/OWNERS
+++ b/vendor/k8s.io/client-go/util/cert/OWNERS
@@ -1,3 +1,5 @@
+# See the OWNERS docs at https://go.k8s.io/owners
+
 approvers:
 - sig-auth-certificates-approvers
 reviewers: