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/pom.xml

@@ -27,6 +27,7 @@
 		<module>spring-boot-sample-actuator-log4j2</module>
 		<module>spring-boot-sample-actuator-noweb</module>
 		<module>spring-boot-sample-actuator-ui</module>
+		<module>spring-boot-sample-actuator-custom-security</module>
 		<module>spring-boot-sample-amqp</module>
 		<module>spring-boot-sample-aop</module>
 		<module>spring-boot-sample-atmosphere</module>