Allow overriding InterfaceDefaultRoute with ips_from_pool template

Currently InterfaceDefaultRoute doesn't affect when ips_from_pool
template is used, and gateway_ip in network_data is always picked up.
This patch allows operators to override default route by the parameter
even when ips_from_pool template is used.

Note that this patch also fixes incorrect handling about subnet in
ips_from_pool template when ipv6 is enabled.

Closes-Bug: #1881768
Change-Id: I2f0f60052c1acfc971fe3d5195f9499e45068396
(cherry picked from commit 396b26fe9b)

network/ports/port_from_pool.j2

@@ -53,9 +53,24 @@ parameters:
     default: 0
     type: number
   {{network.name}}NetCidr:
-    default: {{network.ip_subnet}}
+{%- if network.ipv6 or ipv6_override %}
+    default: "{{network.ipv6_subnet|default(network.ip_subnet|default(""))}}"
+{%- else %}
+    default: "{{network.ip_subnet|default("")}}"
+{%- endif %}
     description: Cidr for the {{network.name_lower}} network.
     type: string
+  {{network.name}}InterfaceDefaultRoute:
+    default: ''
+    description: default route for the {{network.name_lower}} network
+    type: string
+
+conditions:
+  {{network.name_lower}}_interface_default_route_set:
+    not:
+      equals:
+        - {get_param: {{network.name}}InterfaceDefaultRoute}
+        - ''
 
 outputs:
   ip_address:
@@ -86,9 +101,13 @@ outputs:
     value: {{network.mtu|default('1500')}}
   gateway_ip:
     description: Gateway IP of the ports subnet
+    value:
+      if:
+        - {{network.name_lower}}_interface_default_route_set
+        - {get_param: {{network.name}}InterfaceDefaultRoute}
+        -
 {%- if network.ipv6|default(false) and network.gateway_ipv6|default(false) %}
-    value: {{network.gateway_ipv6|default('')}}
+          {{network.gateway_ipv6|default('')}}
 {%- else %}
-    value: {{network.gateway_ip|default('')}}
+          {{network.gateway_ip|default('')}}
 {%- endif %}
-