Make bash completion provide sub-command names even if ValidArgs present

[marckhouzam]

Fixes #1076

This PR makes bash completion also provide sub-command names when ValidArgs or ValidArgsFunction are specified.

Here are details about the completions given:

Cobra's Bash completion:
Original (before custom Go completions of PR #1035):

• ValidArgs XOR sub-commands (which means it gives ValidArgs and if there are none, then it gives sub-commands)

Current:

• ValidArgs XOR ValidArgsFunction XOR sub-commands

With this PR:

• (ValidArgs AND sub-commands) XOR (ValidArgsFunction AND sub-commands)

For info, Cobra's Zsh completion does it yet another way:

• sub-commands XOR ValidArgs

Which which should align eventually.

[jharshman]

+label:kind/enhancement

[Fanduzi]

I want to use ValidArgs and Args to validate args, but I don't want them to appear in the completion information

for example: show command have two subcommad [binlog xtrabackup]

# my-recover show --help
Show mysql backup information

Usage:
  my-recover show [flags]
  my-recover show [command]

Available Commands:
  binlog      show backed up binlog information
  xtrabackup  show xtrabackups information

Flags:
  -h, --help   help for show

I hope to verify the args passed by the user and only receive binlog/xtrabackup

# my-recover show hh
Error: invalid argument "hh" for "my-recover show"

therefore i use [Args and ValidArgs]

	Args:      cobra.MatchAll(cobra.ExactArgs(1), cobra.OnlyValidArgs),
	ValidArgs: []string{"xtrabackup", "binlog"},

But when completing, ValidArgs and subcommand are displayed at the same time

# my-recover show 
binlog                                           binlog      (show backed up binlog information)  xtrabackup                                       xtrabackup  (show xtrabackups information)

Is there any other validation method, or is there any switch to prevent ValidArgs from being displayed in the completion?