Add logging.level to application.properties

E.g.

logging.level.org.springframework: DEBUG
logging.level.org.hibernate: WARN

Fixed gh-788

spring-boot-samples/spring-boot-sample-integration/src/main/resources/application.properties

@@ -1,3 +1,4 @@
 logging.file: /tmp/logs/app.log
+logging.level.org.springframework.integration.file: DEBUG
 service.greeting: Hello
 debug: true

spring-boot-samples/spring-boot-sample-integration/src/main/resources/logback.xml

@@ -1,6 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<configuration>
-	<include resource="org/springframework/boot/logging/logback/base.xml"/>	
-
-	<logger name="org.springframework.integration.file" level="DEBUG" />
-</configuration>