Clarify Installation Guide sections for compute node installation

Add note to compute guide installation section to clarify what
sections are needed to only install additional compute nodes.
Include links to performing basic config steps as well as using
either OpenStack Networking or nova-network.

Change-Id: I1b9568c4765fb5820e24045dbfcd5e834f1894e0
backport: juno
Closes-Bug: 1372492
Merge "Clarify Installation Guide sections for compute node installation"