WIP rewrite of the multinode doc
WIP rewrite of the multinode doc

We need to make sure that this file flows cleanly from one test to the
next, adding small changes each time and explaining them fully.

Change-Id: Ic54e78351394da382934f7fa8df3cc7f22ed421e

Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server