Add phase 1 of validation tightening.

https://github.com/kubernetes/enhancements/blob/master/keps/sig-storage/177-volume-snapshot/tighten-validation-webhook-crd.md

1. Ratcheting validation webhook server image
2. Controller labels invalid objects
3. Unit tests for webhook
4. Deployment README and example deployment method with certs
5. Update top-level README

Racheting validation:
1. webhook is strict on create
2. webhook is strict on updates where the existing object passes strict validation
3. webhook is relaxed on updates where the existing object fails strict validation (allows finalizer removal, status update, deletion, etc)

Additionally the validating wehook server will perform immutability
checks on scenario 2 above.

vendor/github.com/inconshreveable/mousetrap/README.md

@@ -0,0 +1,23 @@
+# mousetrap
+
+mousetrap is a tiny library that answers a single question.
+
+On a Windows machine, was the process invoked by someone double clicking on
+the executable file while browsing in explorer?
+
+### Motivation
+
+Windows developers unfamiliar with command line tools will often "double-click"
+the executable for a tool. Because most CLI tools print the help and then exit
+when invoked without arguments, this is often very frustrating for those users.
+
+mousetrap provides a way to detect these invocations so that you can provide
+more helpful behavior and instructions on how to run the CLI tool. To see what
+this looks like, both from an organizational and a technical perspective, see
+https://inconshreveable.com/09-09-2014/sweat-the-small-stuff/
+
+### The interface
+
+The library exposes a single interface:
+
+    func StartedByExplorer() (bool)