Rework security autoconfiguration

This commit combines security autoconfigurations for
management endpoints and the rest of the application. By default,
if Spring Security is on the classpath, it turns on @EnableWebSecurity.
In the presence of another WebSecurityConfigurerAdapter this backs off
completely. A default AuthenticationManager is also provided with a user
and generated password. This can be turned off by specifying a bean of
type AuthenticationManager, AuthenticationProvider or UserDetailsService.

Closes gh-7958

spring-boot-samples/spring-boot-sample-secure-oauth2/src/main/java/sample/secure/oauth2/AuthenticationConfiguration.java

@@ -0,0 +1,17 @@
+package sample.secure.oauth2;
+
+import org.springframework.context.annotation.Configuration;
+import org.springframework.security.config.annotation.authentication.builders.AuthenticationManagerBuilder;
+import org.springframework.security.config.annotation.authentication.configurers.GlobalAuthenticationConfigurerAdapter;
+
+/**
+ * @author Madhura Bhave
+ */
+@Configuration
+public class AuthenticationConfiguration extends GlobalAuthenticationConfigurerAdapter {
+
+	@Override
+	public void init(AuthenticationManagerBuilder auth) throws Exception {
+		auth.inMemoryAuthentication().withUser("greg").password("turnquist").roles("read");
+	}
+}