Add Scaling section to User Guide
Add Scaling section to User Guide

Scaling container, node section

This section covers current support for container and
node scaling, manual and automatic operation, and
pointers to current discussion.

Partially implements: blueprint user-guide
Change-Id: Iccb9402b664e48deb3a0b020205d3c6d69a96251

ArchiFleKs/magnum,openstack/magnum,ArchiFleKs/magnum,openstack/magnum