Add callback for modifying or inspecting LocalContainerEntityManagerFactoryBean

A callback is added in autoconfig, so that if users inject the EntityManagerFactoryBuilder
into their app and use it to create multiple EntityManagerFactories, they all get the
same deferred DDL behaviour. The deferred DDL can also be disabled by setting
spring.jpa.hibernate.deferDdl=true.

Fixes gh-894

spring-boot-samples/spring-boot-sample-flyway/src/main/java/sample/flyway/SampleFlywayApplication.java

@@ -78,7 +78,7 @@ class Person {
 
 	@Override
 	public String toString() {
-		return "Person [firstName=" + firstName + ", lastname=" + lastName
+		return "Person [firstName=" + firstName + ", lastName=" + lastName
 				+ "]";
 	}
 }