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/flag_ext.go

@@ -0,0 +1,48 @@
+package cli
+
+import "flag"
+
+type extFlag struct {
+	f *flag.Flag
+}
+
+func (e *extFlag) Apply(fs *flag.FlagSet) error {
+	fs.Var(e.f.Value, e.f.Name, e.f.Usage)
+	return nil
+}
+
+func (e *extFlag) Names() []string {
+	return []string{e.f.Name}
+}
+
+func (e *extFlag) IsSet() bool {
+	return false
+}
+
+func (e *extFlag) String() string {
+	return FlagStringer(e)
+}
+
+func (e *extFlag) IsVisible() bool {
+	return true
+}
+
+func (e *extFlag) TakesValue() bool {
+	return false
+}
+
+func (e *extFlag) GetUsage() string {
+	return e.f.Usage
+}
+
+func (e *extFlag) GetValue() string {
+	return e.f.Value.String()
+}
+
+func (e *extFlag) GetDefaultText() string {
+	return e.f.DefValue
+}
+
+func (e *extFlag) GetEnvVars() []string {
+	return nil
+}