Allow scripting the terminal in TUF commands

[ecordell]

This change came in here: 25cd29c

But I can't really tell why. It breaks scripting tuf commands (if you're not running in a terminal, all requests are anonymous to the server).

I think #861 is the proper solution, but for now it's nice to be able to script it via stdin.

[cyli]

Hm... I don't really remember why either, although I have a hazy memory that we decided if not in a terminal we can use env vars. I don't know if I am just making this up after the fact though.

This LGTM, thanks for fixing.

ping @endophage and @riyazdf to see if their memory is better than mine :)

[ecordell]

Can you use ENV vars for setting username/password and not just key passphrases?

[cyli]

@ecordell Ah blah, sorry, you're right the auth info cannot be set via ENV vars.

[riyazdf]

LGTM, thanks @ecordell!

We have a similar !terminal.IsTerminal(int(os.Stdin.Fd())) clause for key passphrases but since we have env vars there it makes sense to keep it as-is :)

[riyazdf]

jenkins failure looks unrelated, kicking off tests again now