xenapi: README.md and embedded localrc updates

This change updates the readme, so that it is easier to get started, and
reflects the actual behavior of DevStack.

Changes in README.md:
 - Link to xenserver download page
 - Remove neutron interface - it is no longer installed by devstack
 - Add appendix with
   - How to use a different ubuntu mirror
   - How to use a proxy for ubuntu
   - How to re-use the created VM
 - Remove run from snapshot section and "do cloudy stuff"

Changes in the Readme-embedded sample localrc:
 - Upload a vhd image and a uec image by default - easier to get started

Change-Id: I13bb8e59ff5367ff7623fe9aa273886a957f81a7
Merge "xenapi: README.md and embedded localrc updates"