Convert CLI to urfave v2

Followed the Migration Guide at https://cli.urfave.org/migrate-v1-to-v2/
The major changes not pointed out in the migration guide are:
- context.Args() no longer produces a []slice, so context.Args().Slice()
  in substitued
- All cli.Global***** are deprecated (the migration guide is somewhat
  unclear on this)

Signed-off-by: Derek Nola <derek.nola@suse.com>

Vendor in urfave cli/v2

Signed-off-by: Derek Nola <derek.nola@suse.com>

Fix NewStringSlice calls

Signed-off-by: Derek Nola <derek.nola@suse.com>

cmd/ctr/commands/commands_windows.go

@@ -17,23 +17,21 @@
 package commands
 
 import (
-	"github.com/urfave/cli"
+	"github.com/urfave/cli/v2"
 )
 
 func init() {
 	ContainerFlags = append(ContainerFlags,
-		cli.Uint64Flag{
+		&cli.Uint64Flag{
 			Name:  "cpu-count",
 			Usage: "Number of CPUs available to the container",
-		},
-		cli.Uint64Flag{
+		}, &cli.Uint64Flag{
 			Name:  "cpu-shares",
 			Usage: "The relative number of CPU shares given to the container relative to other workloads. Between 0 and 10,000.",
-		},
-		cli.Uint64Flag{
+		}, &cli.Uint64Flag{
 			Name:  "cpu-max",
 			Usage: "The number of processor cycles threads in a container can use per 10,000 cycles. Set to a percentage times 100. Between 1 and 10,000",
-		}, cli.StringSliceFlag{
+		}, &cli.StringSliceFlag{
 			Name:  "device",
 			Usage: "Identifier of a device to add to the container  (e.g. class://5B45201D-F2F2-4F3B-85BB-30FF1F953599)",
 		})