Adds docs for settings/configuration.

In addition to thoroughly documenting the settings and config
options for Horizon/OpenStack Dashboard, this clarifies some
misconceptions around how to enable Object Storage/Swift
and Netowrking/Quantum in the OpenStack Dashboard.

Fixes bug 1064412. DocImpact.

Change-Id: I85fc7592c55111f5bc69a7cdab49679eaf81c6ca
Merge "Adds docs for settings/configuration."