Reorganise compute config reference

This patch
* Removes content that is covered by installation (adding group,
   fixing file permissions, multiple compute nodes)
* Removes empty section (hypervisors)
* Merges duplicate content (overview/explanation of nova.conf)
* Flattens section structure, and removes "post-install config"
   section label that was a legacy of previous structure
* Addresses outdated reference to Xen configuration info.

Closes-bug: 1095095
Change-Id: I8922606fe38d30d5ac6288901a866c635c686fbe
Merge "Reorganise compute config reference"