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/spf13/cobra/powershell_completions.go

@@ -0,0 +1,100 @@
+// PowerShell completions are based on the amazing work from clap:
+// https://github.com/clap-rs/clap/blob/3294d18efe5f264d12c9035f404c7d189d4824e1/src/completions/powershell.rs
+//
+// The generated scripts require PowerShell v5.0+ (which comes Windows 10, but
+// can be downloaded separately for windows 7 or 8.1).
+
+package cobra
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+
+	"github.com/spf13/pflag"
+)
+
+var powerShellCompletionTemplate = `using namespace System.Management.Automation
+using namespace System.Management.Automation.Language
+Register-ArgumentCompleter -Native -CommandName '%s' -ScriptBlock {
+    param($wordToComplete, $commandAst, $cursorPosition)
+    $commandElements = $commandAst.CommandElements
+    $command = @(
+        '%s'
+        for ($i = 1; $i -lt $commandElements.Count; $i++) {
+            $element = $commandElements[$i]
+            if ($element -isnot [StringConstantExpressionAst] -or
+                $element.StringConstantType -ne [StringConstantType]::BareWord -or
+                $element.Value.StartsWith('-')) {
+                break
+            }
+            $element.Value
+        }
+    ) -join ';'
+    $completions = @(switch ($command) {%s
+    })
+    $completions.Where{ $_.CompletionText -like "$wordToComplete*" } |
+        Sort-Object -Property ListItemText
+}`
+
+func generatePowerShellSubcommandCases(out io.Writer, cmd *Command, previousCommandName string) {
+	var cmdName string
+	if previousCommandName == "" {
+		cmdName = cmd.Name()
+	} else {
+		cmdName = fmt.Sprintf("%s;%s", previousCommandName, cmd.Name())
+	}
+
+	fmt.Fprintf(out, "\n        '%s' {", cmdName)
+
+	cmd.Flags().VisitAll(func(flag *pflag.Flag) {
+		if nonCompletableFlag(flag) {
+			return
+		}
+		usage := escapeStringForPowerShell(flag.Usage)
+		if len(flag.Shorthand) > 0 {
+			fmt.Fprintf(out, "\n            [CompletionResult]::new('-%s', '%s', [CompletionResultType]::ParameterName, '%s')", flag.Shorthand, flag.Shorthand, usage)
+		}
+		fmt.Fprintf(out, "\n            [CompletionResult]::new('--%s', '%s', [CompletionResultType]::ParameterName, '%s')", flag.Name, flag.Name, usage)
+	})
+
+	for _, subCmd := range cmd.Commands() {
+		usage := escapeStringForPowerShell(subCmd.Short)
+		fmt.Fprintf(out, "\n            [CompletionResult]::new('%s', '%s', [CompletionResultType]::ParameterValue, '%s')", subCmd.Name(), subCmd.Name(), usage)
+	}
+
+	fmt.Fprint(out, "\n            break\n        }")
+
+	for _, subCmd := range cmd.Commands() {
+		generatePowerShellSubcommandCases(out, subCmd, cmdName)
+	}
+}
+
+func escapeStringForPowerShell(s string) string {
+	return strings.Replace(s, "'", "''", -1)
+}
+
+// GenPowerShellCompletion generates PowerShell completion file and writes to the passed writer.
+func (c *Command) GenPowerShellCompletion(w io.Writer) error {
+	buf := new(bytes.Buffer)
+
+	var subCommandCases bytes.Buffer
+	generatePowerShellSubcommandCases(&subCommandCases, c, "")
+	fmt.Fprintf(buf, powerShellCompletionTemplate, c.Name(), c.Name(), subCommandCases.String())
+
+	_, err := buf.WriteTo(w)
+	return err
+}
+
+// GenPowerShellCompletionFile generates PowerShell completion file.
+func (c *Command) GenPowerShellCompletionFile(filename string) error {
+	outFile, err := os.Create(filename)
+	if err != nil {
+		return err
+	}
+	defer outFile.Close()
+
+	return c.GenPowerShellCompletion(outFile)
+}