Add a new chapter for upgrades

This chapter is probably the most-requested document in
all of OpenStack. Let's see if we can do it.

* pre-upgrade testing
* steps to upgrade Compute and Networking grizzly-->havana

Do not merge until testing is done.

Change-Id: Ic27b98c30dec278746c22f434e32ed677247f8a9
Merge "Add a new chapter for upgrades"