Refactor OVNMacAddressNetwork

Move the OVNMacAddressNetwork resource out of the
Networks resource. This is a special network that
we want to create internally.

Having this network nested in the Networks resource
interfer with the work related to creating composable
networks with non-heat tooling.

Change-Id: I5551af42bc87fb8489edbfbd2abadba3dca30b0b
(cherry picked from commit 6362a0205f)

overcloud.j2.yaml

@@ -896,6 +896,9 @@ resources:
     properties:
       CtlplaneNetworkCidrs: {get_attr: [ControlVirtualIP, network, tags]}
 
+  OVNMacAddressNetwork:
+    type: OS::TripleO::OVNMacAddressNetwork
+
 {%- for role in roles %}
   {{role.name}}GroupVars:
     type: OS::Heat::Value