Introduce 'rabbitmq-diagnostics os_env'

[michaelklishin]

Proposed Changes

This introduces a new diagnostics command, rabbitmq-diagnostics os_env.

It prints RabbitMQ-specific environment variables that are set on the target node. Can be used to inspect env variable-based configuration without access to the target host.

Types of Changes

• [ ] Bug fix (non-breaking change which fixes issue #NNNN)
• [x] 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)

Checklist

• [x] I have read the CONTRIBUTING.md document
• [x] I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• [x] All tests pass locally with my changes
• [x] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)
• [x] Any dependent changes have been merged and published in related repositories

[michaelklishin]

Backported to v3.8.x.