Fixes#564
- Correct an issue where if the `cloud` profile is enabled but no Connectors are available,
no `ConnectionFactory` will be created;
- Introduce a property named `spring.cloud.stream.overrideCloudConnectors` that allows
to suppress the use of Cloud connectors in the application and fall back to Spring Boot
configuration. This is provided so that specific binders can be created with arbitrary
connectivity parameters if needed.
- Add documentation.
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