update network and storage overview

 * use the same style for quantum/neutron as for other OpenStack
   services (nova-network, cinder etc.)
 * remove reference to nova-volume which is fully superceded by cinder
 * clarify block and object storage definitions, explain that swift is
   not the only object storage implementation available for OpenStack
 * document HA considerations for Ceph monitors in Logical Setup
Merge pull request #36 from angdraug/ceph-ref-arch-overview

update network and storage overview