Fills in how to configure Distributed Virtual Router for HA neutron

Uses these resources for information:
https://wiki.openstack.org/wiki/Neutron/DVR_L2_Agent
https://wiki.openstack.org/wiki/Neutron/DVR/HowTo
https://docs.google.com/document/d/1iXMAyVMf42FTahExmGdYNGOBFyeA4e74sAO3pvr_RjA/edit

blueprint: create-networking-guide
Change-Id: Ib999326de16c4810cf127e43dac5635c37dd9ce1
Merge "Fills in how to configure Distributed Virtual Router for HA neutron"