Add RootStackName to group_vars

After a Overcloud deployment, /etc/hosts on the undercloud
will be populated with entries for each overcloud node. Since
we use the same tripleo_ansible roles for both the undercloud
and overcloud deployment, the /etc/hosts file on the Director
will be removed by undercloud install / upgrade operations.

This is outlined here:
https://bugzilla.redhat.com/show_bug.cgi?id=1933528

This change adds the RootStackName to the group_vars,
this is then used by the tripleo_host_entries role in
tripleo-ansible to write host entries per stack.

Closes-Bug: #1924751
RHBZ: 1933528
Change-Id: I9e53187f37d41d7180e66db1239b5f9c8846addd

overcloud.j2.yaml

@@ -404,6 +404,10 @@ parameters:
       dns_nameservers attribute will be used.
     type: comma_delimited_list
 
+  RootStackName:
+    description: The name of the stack/plan.
+    type: string
+
 {% for role in roles %}
 {%- if role.deprecated_param_scheduler_hints is defined or role.deprecated_param_extraconfig is defined %}
 {%- if not parameter_groups_defined|default(false) %}
@@ -1313,6 +1317,7 @@ outputs:
           tripleo_network_config_os_net_config_mappings: {get_param: NetConfigDataLookup}
           deployed_server_port_map: {get_param: DeployedServerPortMap}
           tripleo_network_config_override: {get_param: {{role.name}}NetConfigOverride}
+          tripleo_stack_name: {get_param: RootStackName}
 {%- if role.name == 'ComputeOvsDpdk' %}
           num_dpdk_interface_rx_queues: {get_param: NumDpdkInterfaceRxQueues}
 {%- endif %}