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

internal/lockdeps/lockdeps.go

@@ -0,0 +1,8 @@
+package lockdeps
+
+import (
+	// TODO(Sargun): Remove in Go1.13
+	// This is a dep that `go mod tidy` keeps removing, because it's a transitive dep that's pulled in via a test
+	// See: https://github.com/golang/go/issues/29702
+	_ "github.com/prometheus/client_golang/prometheus"
+)