Remove extra output in bash completion

[praveenrewar]

What this PR does / why we need it:

• Do not print Succeeded for help and hidden __complete command
• Use SetOut instead of deprecated SetOutput for setting output for help command.

Which issue(s) this PR fixes:

Fixes #583

Does this PR introduce a user-facing change?

Additional Notes for your reviewer:

Review Checklist:

• Follows the developer guidelines
• Relevant tests are added or updated
• Relevant docs in this repo added or updated
• Relevant carvel.dev docs added or updated in a separate PR and there's
  a link to that PR
• Code is at least as readable and maintainable as it was before this
  change

Additional documentation e.g., Proposal, usage docs, etc.:

[cppforlife]

why do we need to special case this? wouldnt all of these commands (case "help", cobra.ShellCompRequestCmd, cobra.ShellCompNoDescRequestCmd) be running in non-tty env, so Succeeded shouldnt be showing up already?

[praveenrewar]

why do we need to special case this? wouldnt all of these commands (case "help", cobra.ShellCompRequestCmd, cobra.ShellCompNoDescRequestCmd) be running in non-tty env

These commands are a bit special, quoting from the issue itself:

This command is added by Cobra when rootCmd.Execute() is called, which makes it a little trickier to adjust the behaviour for it.

So, right now when we decide which commands should have the default tty behaviour and which should not have it, these commands are not even added to the command tree 😞

[rohitagg2020]

LGTM

[100mik]

The cobrautil path looks much cleaner!
LGTM 🚀