Don't save Hackage credentials without asking first

[decentral1se]

Something for #2159.

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

Here's my manual test logs:

>> :main ["upload", "."]
Getting file list for /home/lwm/hacking/hobby/stack/

Warning: In getPackageArgs: custom-setup in use, but no dependency map present
Could not find custom-setup dep: Cabal
Could not find custom-setup dep: base
Could not find custom-setup dep: filepath
Building sdist tarball for /home/lwm/hacking/hobby/stack/
Checking package 'stack' for common mistakes
Hackage username: foobar
Hackage password: 
Save hackage credentials to file at /home/lwm/.stack/upload/credentials.json [y/n]? bad input
Save hackage credentials to file at /home/lwm/.stack/upload/credentials.json [y/n]? n
NOTE: Avoid this prompt in the future by using: save-hackage-creds: false
Uploading stack-1.5.1.tar.gz... authentication failure
Control.Exception.Safe.throwString called with:

Authentication failure uploading to server
Called from:
  throwString (/home/lwm/hacking/hobby/stack/src/Stack/Upload.hs:181:17 in main:Stack.Upload)

*** Exception: ExitFailure 1

[decentral1se]

Weird CI error - https://travis-ci.org/commercialhaskell/stack/jobs/276544532#L455. Possibly related to #3421. Seems like more of an upstream Travis thing (see travis-ci/travis-ci#7752)? Pretty sure that is unrelated ... will investigate.Rebuilt it but the time out persists :| Fixed 👍

[mgsloan]

LGTM, good stuff!