update post-puppet instructions for jenkins to include moving to /mnt under AWS

I could have chosen to have puppet make this change, but it would have been complicated and error-prone.  Since there is already manual post-puppet customization that needs to be done for a new jenkins install, IMO it's fine to treat the FS move that way too.
* Fixes #502
* Jenkins: http://jenkins.buttonweavers.com:8080/job/buttonmen-cgolubi1/123/