Replace localhost instances with 127.0.0.1

What does this MR do?

I've bumped into a few edge cases in systems where /etc/hosts was missing and the DNS was failing to resolve localhost. This wasn't obvious since listeners are set up using 127.0.0.1 as the address, but localhost is used in a few places.
This changes all instances of localhost to 127.0.0.1 for consistency.

Note: it only affects the tests.

TODO

• I added the Changelog (e.g. Changelog: feature) trailer to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines