Reorganize storage reference architecture

 * Expand Storage Nodes subsection of Logical Setup with HA
   considerations for Ceph and Cinder LVM
 * Fix nesting level of the Neutron vs. nova-network section so that it
   and Savanna Deployment are subsections of Technical Considerations
 * Replace Swift specific Object Storage Deployment section with new
   Storage Architecture that covers all aspects of storage for OpenStack
Ceph deployment diagram for Storage Nodes subsection
Storage reference architecture updates per code review

 * Expanded explanation on when to use dedicated Swift nodes vs
   deploying all Swift services on controllers
 * Describe Ceph RBD support differences between vanilla Havana and
   Mirantis OpenStack 4.0
 * Pre-convert SVG images to PDF with Inkscape, do not pre-convert SVG
   to JPG, let Sphinx choose the best image format (SVG or PDF)