redpanda.yaml redux proposal

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redpanda.yaml is a bit confusing and has many use cases. We should propose a better long term format. This issue will be fleshed out more as a more comprehensive redux is proposed. For starters:

• rpk configuration is separate and distinct from redpanda configuration, and these should be different files. It is surprising that rpk defaults to /etc/redpanda/redpanda.yaml if no other configuration is found.
• If we separate rpk configuration into a separate file, we can collapse the entire rpk section up by one level. The same holds true for the redpanda configuration.
• The way to define listeners is a bit confusing. The original format follows the Kafka format closely, but we can simplify the definitions to make things more understandable to everybody long term.
• Anything "trustfile" should be removed, because trustfile is a Java concept, and the files are actually PEM.

This placeholder issue will be expanded as we plan this more. This issue will affect rpk and redpanda, and we will want to do it in a backwards compatible way, which will be difficult.