Multiple cluster networks

planning/4200-net-topology -- adds mention of multiple L2 networks
for Neutron GRE environments with xrefs for more information

ref-arch/network-concepts/6015-mcn.rst -- explains the implementation;
it is located right after the section that discusses the Logical
Networks.
ref-arch/6010-logical-networks -- I moved the definition of "Internal
network" to the end of the list so that the first four networks listed
correspond to those discussed below.  No content changes were made to
that paragraph.

operations/3500-mcn -- instructions for configuring

terminology/node-group -- define term and xref other docs

file-ref/dnsmasq-template -- ref page for dnsmasq.template file
that defines the DHCP networks used

file-ref/network-1-yaml -- ref page for network_1.yaml where the
networks are configured

release-notes/v6-0/new-features/mcn.rst is the "New Features"
article for the Release Notes.

Change-Id: Id6b23f8036a0b2faaf7f29e64bd7edb55b0ec166
Closes-Bug: 1395638
Merge "Multiple cluster networks"