[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/apimachinery/pkg/api/resource/quantity.go b/vendor/k8s.io/apimachinery/pkg/api/resource/quantity.go
index 54fda58..93a6c0c 100644
--- a/vendor/k8s.io/apimachinery/pkg/api/resource/quantity.go
+++ b/vendor/k8s.io/apimachinery/pkg/api/resource/quantity.go
@@ -584,6 +584,12 @@
 	q.d.Dec.Neg(q.d.Dec)
 }
 
+// Equal checks equality of two Quantities. This is useful for testing with
+// cmp.Equal.
+func (q Quantity) Equal(v Quantity) bool {
+	return q.Cmp(v) == 0
+}
+
 // int64QuantityExpectedBytes is the expected width in bytes of the canonical string representation
 // of most Quantity values.
 const int64QuantityExpectedBytes = 18