Make @ConfigurationProperties available in @PostConstruct

Also means that persistence annotation processing can take advantage
of external properties bound in this way.

spring-boot-samples/spring-boot-sample-data-mongodb/src/test/java/sample/data/mongo/SampleMongoApplicationTests.java

@@ -16,7 +16,8 @@
 
 package sample.data.mongo;
 
-import java.io.IOException;
+import static org.junit.Assert.assertTrue;
+
 import java.net.ConnectException;
 
 import org.junit.Rule;
@@ -24,8 +25,6 @@ import org.junit.Test;
 import org.springframework.boot.test.OutputCapture;
 import org.springframework.core.NestedCheckedException;
 
-import static org.junit.Assert.assertTrue;
-
 /**
  * Tests for {@link SampleMongoApplication}.
  *