Suppress log4j warning.

Workaround to suppress following message at startup.
---
SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/home/onos/.m2/repository/ch/qos/logback/logback-classic/1.0.13/logback-classic-1.0.13.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/home/onos/.m2/repository/org/slf4j/slf4j-log4j12/1.6.1/slf4j-log4j12-1.6.1.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.
SLF4J: Actual binding is of type [ch.qos.logback.classic.util.ContextSelectorStaticBinder]
---

Change-Id: I2bc785550ee1f477c0cb636ef0a9e96330cd351e
diff --git a/pom.xml b/pom.xml
index 9e09386..a39db9d 100644
--- a/pom.xml
+++ b/pom.xml
@@ -591,6 +591,17 @@
       <artifactId>zookeeper</artifactId>
       <version>3.4.6</version>
       <scope>runtime</scope>
+      <!-- Excluding log4j. ONOS uses logback-classic for SLF4J. -->
+      <exclusions>
+        <exclusion>
+          <artifactId>slf4j-log4j12</artifactId>
+          <groupId>org.slf4j</groupId>
+        </exclusion>
+        <exclusion>
+          <artifactId>log4j</artifactId>
+          <groupId>log4j</groupId>
+        </exclusion>
+      </exclusions>
     </dependency>
     <dependency>
       <groupId>org.apache.curator</groupId>