Don't detect CloudPlatform when property is set

Update `CloudPlatform.isActive` to back-off from detection when any
`spring.main.cloud-platform` property is set.

See gh-25455

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/cloud/CloudPlatform.java

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -28,6 +28,7 @@ import org.springframework.core.env.StandardEnvironment;
  *
  * @author Phillip Webb
  * @author Brian Clozel
+ * @author Nguyen Sach
  * @since 1.3.0
  */
 public enum CloudPlatform {
@@ -131,13 +132,15 @@ public enum CloudPlatform {
 
 	};
 
+	private static final String PROPERTY_NAME = "spring.main.cloud-platform";
+
 	/**
 	 * Determines if the platform is active (i.e. the application is running in it).
 	 * @param environment the environment
 	 * @return if the platform is active.
 	 */
 	public boolean isActive(Environment environment) {
-		return isEnforced(environment) || isDetected(environment);
+		return isEnforced(environment) || (isAutoDetectionEnabled(environment) && isDetected(environment));
 	}
 
 	/**
@@ -148,7 +151,7 @@ public enum CloudPlatform {
 	 * @since 2.3.0
 	 */
 	public boolean isEnforced(Environment environment) {
-		String platform = environment.getProperty("spring.main.cloud-platform");
+		String platform = environment.getProperty(PROPERTY_NAME);
 		return name().equalsIgnoreCase(platform);
 	}
 
@@ -161,6 +164,16 @@ public enum CloudPlatform {
 	 */
 	public abstract boolean isDetected(Environment environment);
 
+	/**
+	 * Determines if it is enabled that the platform is detected by looking for
+	 * platform-specific environment variables.
+	 * @param environment the environment
+	 * @return if the platform auto-detection is enabled.
+	 */
+	private boolean isAutoDetectionEnabled(Environment environment) {
+		return environment.getProperty(PROPERTY_NAME) == null;
+	}
+
 	/**
 	 * Returns if the platform is behind a load balancer and uses
 	 * {@literal X-Forwarded-For} headers.

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/cloud/CloudPlatformTests.java

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,7 +18,10 @@ package org.springframework.boot.cloud;
 
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
 
 import org.junit.jupiter.api.Test;
 
@@ -34,6 +37,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  * Tests for {@link CloudPlatform}.
  *
  * @author Phillip Webb
+ * @author Nguyen Sach
  */
 class CloudPlatformTests {
 
@@ -137,6 +141,18 @@ class CloudPlatformTests {
 		assertThat(platform).isEqualTo(CloudPlatform.KUBERNETES);
 	}
 
+	@Test
+	void isActiveWhenNoCloudPlatformIsEnforcedAndHasKubernetesServiceHostAndKubernetesServicePort() {
+		Map<String, Object> envVars = new HashMap<>();
+		envVars.put("EXAMPLE_SERVICE_HOST", "---");
+		envVars.put("EXAMPLE_SERVICE_PORT", "8080");
+		Environment environment = getEnvironmentWithEnvVariables(envVars);
+		((MockEnvironment) environment).setProperty("spring.main.cloud-platform", "none");
+		List<CloudPlatform> activeCloudPlatforms = Stream.of(CloudPlatform.values())
+				.filter((cloudPlatform) -> cloudPlatform.isActive(environment)).collect(Collectors.toList());
+		assertThat(activeCloudPlatforms).containsExactly(CloudPlatform.NONE);
+	}
+
 	private Environment getEnvironmentWithEnvVariables(Map<String, Object> environmentVariables) {
 		MockEnvironment environment = new MockEnvironment();
 		PropertySource<?> propertySource = new SystemEnvironmentPropertySource(