Install fixes, from following the guide.

Some installation guide fixes that appeared when following the install
guide for Ubuntu 12.04
* Add a lead in to the start of the clients chapter
* Allow the glance install to refer to other backends
* Move config file edit commands together in nova controller

Change-Id: I280a95d5d2d71a8d86d51db1fa2a0d5151dbfc74
Merge "Install fixes, from following the guide."