Add per-network routes to NIC templates

This change adds a new {{network.name}}InterfaceRoutes
parameter to network config templates. It takes a list
of routes i.e:
  [{'destination':'10.0.0.0/16', 'nexthop':'10.0.0.1'}]

Co-Authored-By: Harald Jensås <hjensas@redhat.com>
Partial: blueprint tripleo-routed-networks-templates
Depends-On: Ifc5aad7a154c33488a7613c8ee038c92ee6cb1a7
Change-Id: I90aea46d3addab9792c7c9d4feff5c5f61520b9b

net-config-standalone.j2.yaml

@@ -12,6 +12,14 @@ parameters:
     default: ''
     description: IP address/subnet on the {{network.name_lower}} network
     type: string
+  {{network.name}}InterfaceRoutes:
+    default: []
+    description: >
+      Routes for the {{network.name_lower}} network traffic.
+      JSON route e.g. [{'destination':'10.0.0.0/16', 'nexthop':'10.0.0.1'}]
+      Unless the default is changed, the parameter is automatically resolved
+      from the subnet host_routes attribute.
+    type: json
   {%- endfor %}
   ControlPlaneSubnetCidr:
     default: ''