Override CloudPlatform auto-detection with configuration property

[bclozel]

Currenlty the CloudPlatform auto-detection (used by the onCloudPlatformCondition) relies on environment properties detection. In some cases, like Kubernetes, we're strictly looking for environment variables, which are hard to mock for tests purposes.

This issue should introduce a new spring.main.cloud-platform configuration property for overriding the auto-detection and forcing a particular cloud platform.

[bclozel]

This change has been reverted in 05db469.

[wilkinsona]

We've decided to proceed with this after all. See #20620 for some additional context.