Restructured Install Guide

Full outline finished. Keystone, Glance, and most of Nova complete

Changes to Common:
* Separate "Getting Started" content into separate files, so they can be
included individually where needed in the install guide
* separated "Keystone Concepts" so that a smaller subset of that can be
used in the install guide

Change-Id: I583349443685e3022f4c4c1893c2c07d1d2af1d5
Merge "Restructured Install Guide"