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-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -167,8 +167,9 @@ content into your application; rather pick only the properties that you need.
 	spring.jpa.show-sql=true
 	spring.jpa.database-platform=
 	spring.jpa.database=
-	spring.jpa.generate-ddl=
+	spring.jpa.generate-ddl=false # ignored by Hibernate, might be useful for other vendors
 	spring.jpa.hibernate.naming-strategy= # naming classname
+	spring.jpa.hibernate.defer-ddl=true # defer processing of DDL until application is running
 	spring.jpa.hibernate.ddl-auto= # defaults to create-drop for embedded dbs
 
 	# FLYWAY ({sc-spring-boot-autoconfigure}/flyway/FlywayProperties.{sc-ext}[FlywayProperties])

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -1397,8 +1397,10 @@ following to your `application.properties`.
 NOTE: Hibernate's own internal property name for this (if you happen to remember it
 better) is `hibernate.hbm2ddl.auto`. You can set it, along with other Hibernate native
 properties, using `spring.jpa.properties.*` (the prefix is stripped before adding them
-to the entity manager). Also, `spring.jpa.generate-ddl=false` switches off all
-DDL generation.
+to the entity manager). By default the DDL execution (or validation) is deferred until
+the `ApplicationContext` has started. There is also a `spring.jpa.generate-ddl` flag, but
+it is not used if Hibernate autoconfig is active because the `ddl-auto`
+settings are more fine grained.