Add node provider interfaace (#526)

This starts the work of having a `NodeProvider` which is responsible for
providing node details.
It splits the responsibilities of node management off to a new
controller.

The primary change here is to add the framework pieces for node
management and move the VK CLI to use this new controller.

It also adds support for node leases where available. This can be
enabled via the command line (disabled by default), but may fall back if
we find that leaess aren't supported on the cluster.

vendor/github.com/google/go-cmp/cmp/unsafe_panic.go

@@ -0,0 +1,15 @@
+// Copyright 2017, The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE.md file.
+
+// +build purego appengine js
+
+package cmp
+
+import "reflect"
+
+const supportAllowUnexported = false
+
+func unsafeRetrieveField(reflect.Value, reflect.StructField) reflect.Value {
+	panic("unsafeRetrieveField is not implemented")
+}