Improve links and graphics for networking docs

ref-arch
- neutron-intro/0260-neutron-config -- added link to "Logical networks"
- nova-intro/0680-nova-vlan-plan -- added tag, link to "Logical networks"
- deleted table with embedded network drawings; the build tools are having
  fits with this and it just duplicates information in the next two sections.

terminology
- networking -- added xref to "Changing PXE Network Parameters During Installation"
- nova-network -- fixed xref to ref-arch section, added xref to network planning
- VLAN -- added xrefs to ref-arch and user-guide

Change-Id: I67c494ec359f3304ed70eb03b78f4ba070bcafdf