Add @FunctionalSpringBootTest and mini web server

User can run a minimal HTTP app using an app that is a Function
or an ApplicationContextInitializer. Can also test using
@FunctionalSpringBootTest in place of @SpringBootTest.

Add some tests and documentation for functional beans

Make server.address configurable

spring-cloud-function-context/pom.xml

@@ -53,7 +53,7 @@
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-test</artifactId>
-			<scope>test</scope>
+			<optional>true</optional>
 		</dependency>
 		<dependency>
 			<groupId>org.springframework.cloud</groupId>