ZEPPELIN-2530: Zeppelin user impersonation with domain name suffix is…

… failing

### What is this PR for?
Basically what happens is, if a user login using full name with suffix then the user impersonation fails, as the HDFS expects username without the suffix.
This is because the username is passed to underlying components with suffix and got rejected in security layer with IllegalArgumentException

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

### What is the Jira issue?
* [ZEPPELIN-2530](https://issues.apache.org/jira/browse/ZEPPELIN-2530)

### How should this be tested?
 - Enable AD authentication
 - set `activeDirectoryRealm.principalSuffix` in shiro.ini
 - now try to login with the full user name (in my example its zepplintestdomain.com)

### Screenshots (if appropriate)

Before:
<img width="1439" alt="screen shot 2017-05-11 at 7 01 24 pm" src="https://cloud.githubusercontent.com/assets/674497/25951758/44d8adda-367c-11e7-82c1-ecbe2737e13a.png">

After:
<img width="1440" alt="screen shot 2017-05-11 at 7 00 47 pm" src="https://cloud.githubusercontent.com/assets/674497/25951766/47fbc470-367c-11e7-8d14-31465a4db8bf.png">

### Questions:
* Does the licenses files need update? n/a
* Is there breaking changes for older versions? n/a
* Does this needs documentation? n/a

Author: Prabhjyot Singh <[email protected]>

Closes apache#2337 from prabhjyotsingh/ZEPPELIN-2530 and squashes the following commits:

f135eb4 [Prabhjyot Singh] validate user string for null/empty before sending it to AD server
5a02759 [Prabhjyot Singh] ZEPPELIN-2530: Zeppelin user impersonation with domain name suffix is failing

zeppelin-server/src/main/java/org/apache/zeppelin/realm/ActiveDirectoryGroupRealm.java

@@ -186,7 +186,7 @@ protected AuthenticationInfo queryForAuthenticationInfo(
     LdapContext ctx = null;
     try {
       String userPrincipalName = upToken.getUsername();
-      if (userPrincipalName == null) {
+      if (!isValidPrincipalName(userPrincipalName)) {
         return null;
       }
       if (this.principalSuffix != null && userPrincipalName.indexOf('@') < 0) {
@@ -201,7 +201,24 @@ protected AuthenticationInfo queryForAuthenticationInfo(
     return buildAuthenticationInfo(upToken.getUsername(), upToken.getPassword());
   }
 
+  private Boolean isValidPrincipalName(String userPrincipalName) {
+    if (userPrincipalName != null) {
+      if (StringUtils.isNotEmpty(userPrincipalName) && userPrincipalName.contains("@")) {
+        String userPrincipalWithoutDomain = userPrincipalName.split("@")[0].trim();
+        if (StringUtils.isNotEmpty(userPrincipalWithoutDomain)) {
+          return true;
+        }
+      } else if (StringUtils.isNotEmpty(userPrincipalName)) {
+        return true;
+      }
+    }
+    return false;
+  }
+
   protected AuthenticationInfo buildAuthenticationInfo(String username, char[] password) {
+    if (this.principalSuffix != null && username.indexOf('@') > 1) {
+      username = username.split("@")[0];
+    }
     return new SimpleAuthenticationInfo(username, password, getName());
   }