Re-add NovaVncProxyNetwork to service_net_map.j2.yaml

Icef2481b65b41b524ad44eeecfbee4451006e1d2 removed the NovaVncProxyNetwork
from network/service_net_map.j2.yaml, but due to how hieradata is
being rendered via tripleo-hieradata, the hieradata for the no-vnc-proxy
service falls back to the control plane network.

Change-Id: I595294956a4a7a0e2280c685ac1d2543edbc32f2
Related-Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1960401
(cherry picked from commit 36648cf)
(cherry picked from commit 326452e)

network/service_net_map.j2.yaml

@@ -56,6 +56,7 @@ parameters:
       NovaApiNetwork: {{ _service_nets.get('internal_api', 'ctlplane') }}
       PlacementNetwork: {{ _service_nets.get('internal_api', 'ctlplane') }}
       NovaMetadataNetwork: {{ _service_nets.get('internal_api', 'ctlplane') }}
+      NovaVncProxyNetwork: {{ _service_nets.get('internal_api', 'ctlplane') }}
       NovaLibvirtNetwork: {{ _service_nets.get('internal_api', 'ctlplane') }}
       NovajoinNetwork: {{ _service_nets.get('internal_api', 'ctlplane') }}
       SwiftStorageNetwork: {{ _service_nets.get('storage_mgmt', 'ctlplane') }}