Ensure the AuthenticationManager is created when needed

There was too much state really in the old implementation of
AuthenticationManagerConfiguration, and it was leading occasionally
to null pointers when method A assumed that method B had already
been called and it hadn't. This change manages to concentrate all the
references to an AuthenticationManagerBuilder into a single method
call, removoing the need for storing it at all.

Fixes gh-1556

spring-boot-samples/spring-boot-sample-web-secure/src/main/java/sample/ui/secure/SampleWebSecureApplication.java

@@ -101,6 +101,7 @@ public class SampleWebSecureApplication extends WebMvcConfigurerAdapter {
 					.password("admin").roles("ADMIN", "USER").and().withUser("user")
 					.password("user").roles("USER");
 		}
+
 	}
 
 }