Jarv/ssh motd

This disables some of the default motd scripts and adds a security banner:

```
$ ssh jarv.m.sandbox.edx.org
Welcome to Ubuntu 12.04.2 LTS (GNU/Linux 3.2.0-40-virtual x86_64)

*** /dev/xvda1 will be checked for errors at next reboot ***

*** System restart required ***
******************************************************************* 
*                                                                 *  
*   _   _| |\ \/ /  This system is for the use of authorized      * 
* / -_) _` | >  <   users only.  Usage of this system may be      *
* \___\__,_|/_/\_\  monitoried and recorded by system personnel.  *
*                                                                 *
* Anyone using this system expressly consents to such monitoring  *
* and is advised that if such monitoring reveals possible         *
* evidence of criminal activity, system personnel may provide the *
* evidence from such monitoring to law enforcement officials.     *
*                                                                 *
*******************************************************************
Last login: Tue Feb 11 19:12:25 2014 from terminus.mit.edu
```