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_string.go

@@ -0,0 +1,100 @@
+package cli
+
+import (
+	"flag"
+	"fmt"
+)
+
+// TakesValue returns true of the flag takes a value, otherwise false
+func (f *StringFlag) TakesValue() bool {
+	return true
+}
+
+// GetUsage returns the usage string for the flag
+func (f *StringFlag) GetUsage() string {
+	return f.Usage
+}
+
+// GetCategory returns the category for the flag
+func (f *StringFlag) GetCategory() string {
+	return f.Category
+}
+
+// GetValue returns the flags value as string representation and an empty
+// string if the flag takes no value at all.
+func (f *StringFlag) GetValue() string {
+	return f.Value
+}
+
+// GetDefaultText returns the default text for this flag
+func (f *StringFlag) GetDefaultText() string {
+	if f.DefaultText != "" {
+		return f.DefaultText
+	}
+	val := f.Value
+	if f.defaultValueSet {
+		val = f.defaultValue
+	}
+
+	if val == "" {
+		return val
+	}
+	return fmt.Sprintf("%q", val)
+}
+
+// GetEnvVars returns the env vars for this flag
+func (f *StringFlag) GetEnvVars() []string {
+	return f.EnvVars
+}
+
+// Apply populates the flag given the flag set and environment
+func (f *StringFlag) Apply(set *flag.FlagSet) error {
+	// set default value so that environment wont be able to overwrite it
+	f.defaultValue = f.Value
+	f.defaultValueSet = true
+
+	if val, _, found := flagFromEnvOrFile(f.EnvVars, f.FilePath); found {
+		f.Value = val
+		f.HasBeenSet = true
+	}
+
+	for _, name := range f.Names() {
+		if f.Destination != nil {
+			set.StringVar(f.Destination, name, f.Value, f.Usage)
+			continue
+		}
+		set.String(name, f.Value, f.Usage)
+	}
+
+	return nil
+}
+
+// Get returns the flag’s value in the given Context.
+func (f *StringFlag) Get(ctx *Context) string {
+	return ctx.String(f.Name)
+}
+
+// RunAction executes flag action if set
+func (f *StringFlag) RunAction(c *Context) error {
+	if f.Action != nil {
+		return f.Action(c, c.String(f.Name))
+	}
+
+	return nil
+}
+
+// String looks up the value of a local StringFlag, returns
+// "" if not found
+func (cCtx *Context) String(name string) string {
+	if fs := cCtx.lookupFlagSet(name); fs != nil {
+		return lookupString(name, fs)
+	}
+	return ""
+}
+
+func lookupString(name string, set *flag.FlagSet) string {
+	if f := set.Lookup(name); f != nil {
+		return f.Value.String()
+	}
+	return ""
+}