Document how to switch to new master under chef-sync

This is a WIP since I have not tested the steps below. I would love help from @stevendanna or @scotthain or others familiar with replication that the steps in `/includes_install_server_replication_new_master.rst` seem about right before I go off and test it.

Also, I've heard that replica to replica sync is feasible, so updated scenarios (and added use cases). Let me know if that needs testing.

Thanks,

--Peter
Stopping the old master is only necessary if they want to ensure no writes go there. It isn't strictly necessary to point a replica at a different master.