fix broken integration with Swift when Kerberos enabled

related issue in hadoop jira [0]

[0] https://issues.apache.org/jira/browse/HADOOP-10326
Change-Id: Iecdff57454ff91187fba077af73211faf2100d57

hadoop-swiftfs/src/main/java/org/apache/hadoop/fs/swift/snative/SwiftNativeFileSystem.java

@@ -150,6 +150,11 @@ public class SwiftNativeFileSystem extends FileSystem {
     return workingDir;
   }
 
+  @Override
+  public String getCanonicalServiceName() {
+    return null;
+  }
+
   /**
    * @param dir user working directory
    */