Restructured Compute chapter in Cloud Admin Guide

Reduced number of upper-level sections. Renamed 'Volumes' section to 'Manage Volumes'. Shifted a number of admin sections into 'System administration', and moved conceptual sections into 'System architecture.

Partial-Bug: #1290687

Change-Id: I65018afc248737db59b1a839f4d3e76e86bb649a
Typo: replace cGroups with cgroups

Trivial typo fix. cgroups are not cGroups.

Change-Id: If409a47125ccdb91c250cc64aba5e239bf5cca32
Merge "Typo: replace cGroups with cgroups"
Merge "Restructured Compute chapter in Cloud Admin Guide"