Fully qualified username can not be used for login when principalSuffix is defined

• PR #1784

When Zeppelin is configured against Active Directory for user authentication, by default, fully qualified user names are allowed. E.g. user1@DOMAIN.COM

User can be spared from typing the long domain name each time by specifying "activeDirectoryGroupRealm.principalSuffix = @DOMAIN.COM" in shiro.ini. Now user can just specify short username i.e. "user1" during login.

Problem:
When principalSuffix is defined in shiro.ini, only the short username are allowed and any attempt with fully qualified user name will result in the login error.

Excepted Behavior:
Zeppelin UI should allow both username formats (short and fully qualified) when principalSuffix is defined