Make it possible to configure per-node runtime parameter limits

[illotum]

Proposed Changes

Introduces an arbitrary per-host runtime parameter limit, e.g.

runtime_parameters.limits.shovel = 0
runtime_parameters.limits.federation = 100

The limit can be set to any value, negative integers treated as "unlimited". There are no sanity checks, and I expect it to be used only by power-users.

Main purpose is resource management per #7856, although this hypothetically can be used to disable certain broker features outright.

Types of Changes

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

[michaelklishin]

We should use more realistic parameter names in tests, and clarify what exactly "host" means
in the message, e.g. is it a virtual host? a node?