Upgrade K8s

* Upgrade k8s.io/api
   go get k8s.io/api@kubernetes-1.15.2
 * Upgrade k8s.io/apimachinery
   go get k8s.io/apimachinery@kubernetes-1.15.2
 * Upgrade kubernetes-1.15.2
   go get k8s.io/client-go@kubernetes-1.15.2
 * Upgrade kk8s.io/kubernetes to v1.15.2
   go get k8s.io/kubernetes@v1.15.2

This also locks the the dependency for
github.com/prometheus/client_golang/prometheus due to a golang bug, and to
please the validation scripts.

The replaces were generated by:
go get k8s.io/kubernetes@v1.15.2 2> fail
for i in $(cat fail|grep unknown|cut -f1 -d@|cut -f2 -d" ")
  do echo "replace ${i} => ${i} kubernetes-1.15.2"
done

cmd/virtual-kubelet/main.go

@@ -38,7 +38,7 @@ import (
 var (
 	buildVersion = "N/A"
 	buildTime    = "N/A"
-	k8sVersion   = "v1.13.7" // This should follow the version of k8s.io/kubernetes we are importing
+	k8sVersion   = "v1.15.2" // This should follow the version of k8s.io/kubernetes we are importing
 )
 
 func main() {