Fix multiple-nics network config ExternalMtu

The external network is used for the external bridge
in OVN and DVR usecase. Controlled by external_bridge
tag in the role.

For this to work we also need to include the ExternalMtu
parameter when external_bridge tag is present.

Closes-Bug: #1847360
Change-Id: I93d5bfb540d298765ebc35d7596b214271f07dca

network/config/multiple-nics/role.role.j2.yaml

@@ -74,6 +74,14 @@ parameters:
       from the subnet host_routes attribute.
     type: json
 {%- endfor %}
+{% for network in networks if network.name == "External" and 'external_bridge' in role.tags and not network.name in role.networks %}
+  {{network.name}}Mtu:
+    default: {{network.mtu|default('1500')}}
+    description: The maximum transmission unit (MTU) size(in bytes) that is
+      guaranteed to pass through the data path of the segments in the
+      {{network.name}} network.
+    type: number
+{% endfor %}
   DnsServers: # Override this via parameter_defaults
     default: []
     description: >

releasenotes/notes/fix-external-bridge-mtu-multiple-nics-config-43b8dc459b5694b2.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    The `multiple-nics` network template example was rendered without the
+    ``ExternalMtu`` parameter when the role tag ``external_bridge`` was
+    set. This caused the deployment to fail with parameter not provided
+    error. Bug: `1847360 <https://bugs.launchpad.net/tripleo/+bug/1847360>`_.