Minor: Don't use Mnesia in rabbitmq_mqtt/test/processor_SUITE

[gomoripeti]

Proposed Changes

Soon Mnesia will be gone from RabbitMQ, so better make the test suite metadata store agnostic. (The suite contains unit tests so mocking on a higher level seemed acceptable)

Also refactored mqtt_processor get_vhost functions, in order to clarify preference of different methods. This commit does
not change functionality. (I wanted to prepare an actual change in this area but abandoned the idea. But I think this refactoring makes the code a bit more readable

This highlighted some differences (which I initially thought as inconsistencies, but after digging the history they seem to be intentional and the behaviour is documented in the processor_SUITE)

• If both User/Password and SslLogin are provided, in
  check_credentials User/Password takes precedence while in
  get_vhost SslLogin

• If SslLogin is provided (but no mapping is found) vhost from port
  mapping has precedence over vhost from UserName, while in case of
  no ssl it is the other way around.

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[michaelklishin]

Using Mnesia less in the suites where we do not test a Mnesia-specific behavior or Mnesia compatibility per se — and here we don't — makes sense to me, even though I am generally not a fan of mocks.

[ansd]

Thank you @gomoripeti, the code is simpler to read now after your refactor!