Allow special purpose container interfaces

Some use cases such as public facing dns servers for Designate
are difficult to deploy with the dynamic inventory as they have
a hard requirement for fixed IP on certain interfaces. An additional
variable allows the deployer to create these special interfaces.

Change-Id: I4f0ac58f2d5c19fea1606fe1fbb011bb1a36f7b4

releasenotes/notes/container-extra-networks-c74119ba6a559a59.yaml

@@ -0,0 +1,8 @@
+---
+features:
+  - If defined in applicable host or group vars the variable
+    ``container_extra_networks`` will be merged with the existing
+    ``container_networks`` from the dynamic inventory. This allows a deployer
+    to specify special interfaces which may be unique to an indivdual
+    container. An example use for this feature would be applying known
+    fixed IP addresses to public interfaces on BIND servers for designate.

vars/main.yml

@@ -14,6 +14,7 @@
 # limitations under the License.
 
 # A set of container networks used within the LXC containers. This information
-# is sourced from the assumed "container_networks" and "lxc_container_networks"
-# variables and combined into a single immutable hash.
-lxc_container_networks_combined: "{{ container_networks | default({}) | combine(lxc_container_networks) }}"
+# is sourced from the assumed "container_networks", "container_extra_networks"
+# and "lxc_container_networks" variables and combined into a single
+# immutable hash.
+lxc_container_networks_combined: "{{ container_networks | default({}) | combine(container_extra_networks | default({})) | combine(lxc_container_networks) }}"