ZEPPELIN-3357. Livy security mode is broken

### What is this PR for?
Straightforward fix for livy security issue.

### What type of PR is it?
[Bug Fix]

### Todos
* [ ] - Task

### What is the Jira issue?
* https://issues.apache.org/jira/browse/ZEPPELIN-3357

### How should this be tested?
* CI pass

### Screenshots (if appropriate)

### Questions:
* Does the licenses files need update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No

Author: Jeff Zhang <zjffdu@apache.org>

Closes apache#2887 from zjffdu/ZEPPELIN-3357 and squashes the following commits:

2fbbbeb [Jeff Zhang] ZEPPELIN-3357. Livy security mode is broken

livy/src/main/java/org/apache/zeppelin/livy/BaseLivyInterpreter.java

@@ -662,11 +662,12 @@ public Principal getUserPrincipal() {
       } else {
         restTemplate = new KerberosRestTemplate(keytabLocation, principal, httpClient);
       }
-    }
-    if (httpClient == null) {
-      restTemplate = new RestTemplate();
     } else {
-      restTemplate = new RestTemplate(new HttpComponentsClientHttpRequestFactory(httpClient));
+      if (httpClient == null) {
+        restTemplate = new RestTemplate();
+      } else {
+        restTemplate = new RestTemplate(new HttpComponentsClientHttpRequestFactory(httpClient));
+      }
     }
     restTemplate.getMessageConverters().add(0, new StringHttpMessageConverter(Charset.forName("UTF-8")));
     return restTemplate;