[#89] Added test for zipkin integration

spring-cloud-sleuth-samples/spring-cloud-sleuth-sample-zipkin/src/main/resources/logback.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<configuration>
+    <include resource="org/springframework/boot/logging/logback/base.xml"/>
+    <logger name="org.springframework.cloud.sleuth" level="DEBUG"/>
+    <root level="INFO">
+        <appender-ref ref="CONSOLE" />
+        <appender-ref ref="FILE" />
+    </root>
+</configuration>