Add documentation on Zeppelin Shiro's abliity to configure secure cookie

As per AMBARI-21675, secure flag attribute must be set on session cookies containing session tokens for Zeppelin Application. Our doc page on Shiro Authentication (http://zeppelin.apache.org/docs/0.8.0-SNAPSHOT/setup/security/shiro_authentication.html) should talk about this as well.
This bug is to track the doc changes required.