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>

go.mod

@@ -54,7 +54,7 @@ require (
 	github.com/sirupsen/logrus v1.9.3
 	github.com/stretchr/testify v1.8.4
 	github.com/tchap/go-patricia/v2 v2.3.1
-	github.com/urfave/cli v1.22.14
+	github.com/urfave/cli/v2 v2.27.1
 	github.com/vishvananda/netlink v1.2.1-beta.2
 	go.etcd.io/bbolt v1.3.8
 	go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc v0.48.0
@@ -113,6 +113,7 @@ require (
 	github.com/russross/blackfriday/v2 v2.1.0 // indirect
 	github.com/syndtr/gocapability v0.0.0-20200815063812-42c35b437635 // indirect
 	github.com/vishvananda/netns v0.0.4 // indirect
+	github.com/xrash/smetrics v0.0.0-20201216005158-039620a65673 // indirect
 	go.opencensus.io v0.24.0 // indirect
 	go.opentelemetry.io/otel/metric v1.23.1 // indirect
 	go.opentelemetry.io/proto/otlp v1.1.0 // indirect