Zeppelin auth mechanisms (LDAP or password based) should be mutually exclusive

Problem:
When any external authentication (like LDAP/AD) is enabled for Zeppelin, the default password-based authentication could still be configured in addition to that. This makes space for backdoor in Zeppelin where user can still get in using the local username/password.

Workaround:
Currently, the workaround is to make sure that [users] is removed from shiro.ini to stop anyone logging using local username/password.

Proposed Solution:
Zeppelin shouldn't allow specifying [users] section in shiro.ini when it is configured to authenticate with LDAP/AD.