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>

vendor/github.com/urfave/cli/v2/.gitignore

@@ -0,0 +1,14 @@
+*.coverprofile
+*.exe
+*.orig
+.*envrc
+.envrc
+.idea
+# goimports is installed here if not available
+/.local/
+/site/
+coverage.txt
+internal/*/built-example
+vendor
+/cmd/urfave-cli-genflags/urfave-cli-genflags
+*.exe