Make PingTestGatewayIPsMap a map of flatten lists

PingTestGatewayIPsMap elements may contain list of lists, causing failures
on roles that iterate over them. See [1] and #1950528 for more info.

[1] https://review.opendev.org/c/openstack/tripleo-ansible/+/817500

Closes-bug: #1950528
Change-Id: Idb70c822f01f808871a53689edfa2edf52e59e54
Signed-off-by: Douglas Viroel <dviroel@redhat.com>

overcloud.j2.yaml

@@ -1197,7 +1197,7 @@ resources:
 {%- for role in roles %}
         {{role.name}}:
           yaql:
-            expression: list($.data.where($ != null))
+            expression: list($.data.where($ != null)).flatten()
             data:
               - {get_attr: [Networks, net_attributes_map, ctlplane, subnets, {get_param: {{role.name}}ControlPlaneSubnet}, gateway_ip]}
     {%- for network in networks %}