Add parameter for UndercloudNetConfigOverride

This can be used to inject a custom Json blob containing
os-net-config template data for the Undercloud installation.
The intent is to be able to use this alongside of the
net_config_override setting in tripleoclient when installing
the undercloud.

Change-Id: I322570ae958f8e483b8b2a0ad40d1ee83d084372

net-config-undercloud.j2.yaml

@@ -31,6 +31,16 @@ parameters:
     type: number
     constraints:
       - range: { min: 1000, max: 65536 }
+  UndercloudNetConfigOverride:
+    default: {}
+    description: Custom JSON data to be used to override the os-net-config data in this template. This is meant to be used by net_config_override parameter in tripleoclient to provide an easy means to pass in custom net configs for the Undercloud.
+    type: json
+
+conditions:
+  undercloud_net_config_override:
+    not:
+      equals: [{get_param: UndercloudNetConfigOverride}, {}]
+
 resources:
   OsNetConfigImpl:
     type: OS::Heat::SoftwareConfig
@@ -45,7 +55,10 @@ resources:
             get_file: network/scripts/run-os-net-config.sh
           params:
             $network_config:
-              network_config:
+              if:
+              - undercloud_net_config_override
+              - {get_param: UndercloudNetConfigOverride}
+              - network_config:
                 - type: ovs_bridge
                   name: br-ctlplane
                   use_dhcp: false