Allow to override hostname path within EC2 discovery response, AWS

[illotum]

Proposed Changes

Add config to override location of the hostname within EC2 instance description. It is targeting cases where user has multiple interfaces on the instance, but is generic by design.

This also fixes list coercion within common discovery code and some unit tests that were ignored previously.

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• If relevant, I have added necessary documentation to https://github.com/rabbitmq/rabbitmq-website
• If relevant, I have added this change to the first version(s) in release-notes that I expect to introduce it

Further Comments

[michaelklishin]

@illotum do you think the fixed coercion part should be backported to v3.12.x and v3.11.x? If so, please submit a separate PR :)

[illotum]

Sure, it's unlikely to come up within v3.11 lifetime, but doesn't hurt!

[michaelklishin]

@illotum the idea is to backport what can be safely backpored, if it's only to v3.12.x, no objections from me. v3.11.x is reaching its maturity and going out of general support by the end of this month.