Stop exposing a duplicate web extension for the env endpoint

An actuator endpoint is designed around a single class flagged with
`@Endpoint`. Such endpoint can have some of its operations refined with
a given tech (for instance via a `EndpointWebExtension` for web specific
concerns).

This design mandates that an endpoint (and its tech specific refinements
are managed centrally to form a unified contract).

As Spring Cloud is extending the standard Spring Boot's `env` endpoint,
this commit introduces a WritableEnvironmentEndpoint with a web
extension.

Closes gh-367
Closes gh-354

spring-cloud-context/src/test/java/org/springframework/cloud/context/environment/EnvironmentManagerIntegrationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2006-2017 the original author or authors.
+ * Copyright 2006-2018 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.
@@ -18,6 +18,7 @@ package org.springframework.cloud.context.environment;
 
 import java.util.HashMap;
 import java.util.Map;
+import java.util.UUID;
 
 import javax.servlet.ServletException;
 
@@ -44,6 +45,7 @@ import org.springframework.web.context.WebApplicationContext;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.fail;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.post;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
@@ -108,6 +110,12 @@ public class EnvironmentManagerIntegrationTests {
 		assertEquals(0, properties.getDelay());
 	}
 
+	@Test
+	public void coreWebExtensionAvailable() throws Exception {
+		this.mvc.perform(get(BASE_PATH + "/env/" + UUID.randomUUID().toString()))
+				.andExpect(status().isNotFound());
+	}
+
 	@Configuration
 	@EnableAutoConfiguration
 	protected static class TestConfiguration {