Document container_extra_networks usage

Change-Id: Ib17b0817effeee07eb0c74d5e994fa7ae1ba74e3

doc/source/reference/configuration/advanced-config.rst

@@ -15,4 +15,5 @@ installation of OpenStack that may include their own software.
 
    using-overrides
    extra-python-software
+   extra-networks
    extending-osa

doc/source/reference/configuration/extra-networks.rst

@@ -0,0 +1,62 @@
+Adding extra network to container
+=================================
+
+In some cases it may be useful to have an ability to add extra network
+interface for some container group (or just a single container). As an example
+this can be used for applying known fixed IP address from another network for
+Designate service. We will show futher configuration based on this example.
+Let's assume, that this network is 10.0.20.0/24 which is reachable through
+`br-dns` interface.
+
+To add new interface with that network into dessignate containers, we need to
+do several actions in ``openstack_user_config.yml``.
+
+.. note::
+
+   You may find detailed example of `openstack_user_config.yml` configuration
+   in section :ref:`openstack-user-config-reference`.
+
+* Add this network in ``cidr_networks``:
+
+    .. code-block:: yaml
+
+      cidr_networks:
+        container: 172.29.236.0/22
+        tunnel: 172.29.240.0/22
+        storage: 172.29.244.0/22
+        designate: 10.0.20.0/24
+
+* Describe network in ``provider_networks``:
+
+    .. code-block:: yaml
+
+        global_overrides:
+          provider_networks:
+            - network:
+              container_bridge: "br-dns"
+              container_type: "veth"
+              container_interface: "eth5"
+              ip_from_q: "designate"
+              type: "veth"
+              group_binds:
+                - dnsaas_hosts
+
+* Define override for containers
+
+    .. note::
+
+      Adding gateway key will create default route inside container through it
+
+    .. code-block:: yaml
+
+        dnsaas_hosts:
+          aio1:
+            ip: 172.29.236.100
+            container_vars:
+              container_extra_networks:
+                dns_address:
+                  bridge: br-dns
+                  interface: eth5
+                  address: 10.0.20.100
+                  netmask: 255.255.255.0
+                  gateway: 10.0.20.1