Fix octavia doesn't set subnet gateway_ip

the gateway_ip should be in subnet dict.

Closes-Bug: #1949260
Change-Id: Iaa3fc69ee7954b5b202b0a6052c30bfb1780e84b
(cherry picked from commit d7706aa)

Author: wchy1001

ansible/roles/octavia/tasks/prepare.yml

@@ -126,7 +126,7 @@
       allocation_pool_end: "{{ octavia_amp_network['subnet']['allocation_pool_end'] | default(omit, true) }}"
       enable_dhcp: "{{ octavia_amp_network['subnet']['enable_dhcp'] | default(omit) }}"
       no_gateway_ip: "{{ octavia_amp_network['subnet']['no_gateway_ip'] | default(omit) }}"
-      gateway_ip: "{{ octavia_amp_network['gateway_ip'] | default(omit, true) }}"
+      gateway_ip: "{{ octavia_amp_network['subnet']['gateway_ip'] | default(omit, true) }}"
       ip_version: "{{ octavia_amp_network['subnet']['ip_version'] | default(omit) }}"
       ipv6_address_mode: "{{ octavia_amp_network['subnet']['ipv6_address_mode'] | default(omit) }}"
       ipv6_ra_mode: "{{ octavia_amp_network['subnet']['ipv6_ra_mode'] | default(omit) }}"

releasenotes/notes/bug-1949260-34d82ecd677dd8ff.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fix octavia role doesn't set the amphora network's gateway_ip
+    `LP#1949260 <https://bugs.launchpad.net/kolla-ansible/+bug/1949260>`__