openstack-ha: Networking Highly Available

Active / Passive section:
New node: network-controller
* neutron-l3-agent
* neutron-dhcp-agent
* neutron-metadata-agent

Active / Active section:
*neutron-dhcp-agent: redirect to OpenStack Networking guide
*neutron-l3-agent and neutron-metadata-agent: redirect to Active / Passive
section since they are not HA.

implements blueprint improve-high-availability-support
Change-Id: Ie7e4f917f6c7cca05303275cd90e8415e323dfd2
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>