Introduce {{role.name}}ExtraGroupVars

{{role.name}}ExtraGroupVars (e.g. ControllerExtraGroupVars) is a new
interface to override any Ansible group var for a specific role.

If the group var was already defined in a service, the ExtraGroupVars
will override it.

Depends-On: https://review.opendev.org/717170
Change-Id: I3115ea3b1bd14979bad5e2ed45e91107ee969ca9
(cherry picked from commit e5cc47f9e2)

overcloud.j2.yaml

@@ -272,6 +272,11 @@ parameters:
     type: json
     description: Optional Role Specific parameters to be provided to service
     default: {}
+
+  {{role.name}}ExtraGroupVars:
+    type: json
+    description: Optional extra Ansible group vars
+    default: {}
 {% endfor %}
 
   # Identifiers to trigger tasks on nodes
@@ -1198,6 +1203,7 @@ outputs:
           neutron_physical_bridge_name: {get_param: NeutronPhysicalBridge}
           neutron_public_interface_name: {get_param: NeutronPublicInterface}
           network_deployment_actions: {get_attr: [{{role.name}}NetworkDeploymentActionsValue, value]}
+        - {get_param: {{role.name}}ExtraGroupVars}
 {%- endfor %}
   RoleNetHostnameMap:
     description: Mapping of each network to a list of hostnames for each role

releasenotes/notes/extra_group_vars-aafa71945882442f.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    Introduce "{{role.name}}ExtraGroupVars" which allows to define
+    a dictionary of Ansible group vars per role.
+    These extra group vars will override any pre-defined group var from a
+    service.