Only run cellv2 host discovery on default cell

While it would be not an issue to run the host discovery on sub
sequent deploy runs, on the initial cell deplouy the discovery
fails because the cell is not yet defined in nova as this is
a manual post task after the initial cell deployment [1]

[1] https://docs.openstack.org/tripleo-docs/latest/install/advanced_deployment/deploy_cellv2.html#create-the-cell-and-discover-compute-nodes

Change-Id: I42451a8b4dcc690319439415570ce36e5c5333c3
Closes-bug: #1836915

deployment/nova/nova-compute-common-container-puppet.yaml

@@ -32,6 +32,10 @@ parameters:
     default: {}
     description: Parameters specific to the role
     type: json
+  NovaAdditionalCell:
+    default: false
+    description: Whether this is an cell additional to the default cell.
+    type: boolean
 
 outputs:
   container_config_scripts:
@@ -52,21 +56,28 @@ outputs:
     value: &nova_compute_common_deploy_steps_tasks
       - when: step|int == 5
         block:
-          - name: discover via nova_compute?
+          - name: is additonal Cell?
             set_fact:
-              delegate_host: "{{ groups['nova_compute'][0] }}"
+              nova_additional_cell: {get_param: NovaAdditionalCell}
+          - name: discover nodes if it is not an additional cell
             when:
-              - groups['nova_compute'] is defined and (groups['nova_compute']|length>0)
-          - name: discover via nova_ironic?
-            set_fact:
-              delegate_host: "{{ groups['nova_ironic'][0] }}"
-            when:
-              - delegate_host is not defined
-              - groups['nova_ironic'] is defined and (groups['nova_ironic']|length>0)
-          - name: Discovering nova hosts
-            command: "{{ container_cli }} exec nova_compute nova-manage cell_v2 discover_hosts --by-service"
-            become: true
-            changed_when: False
-            when:
-            - delegate_host is defined
-            - inventory_hostname == delegate_host
+              - not nova_additional_cell|bool
+            block:
+              - name: discover via nova_compute?
+                set_fact:
+                  delegate_host: "{{ groups['nova_compute'][0] }}"
+                when:
+                  - groups['nova_compute'] is defined and (groups['nova_compute']|length>0)
+              - name: discover via nova_ironic?
+                set_fact:
+                  delegate_host: "{{ groups['nova_ironic'][0] }}"
+                when:
+                  - delegate_host is not defined
+                  - groups['nova_ironic'] is defined and (groups['nova_ironic']|length>0)
+              - name: Discovering nova hosts
+                command: "{{ container_cli }} exec nova_compute nova-manage cell_v2 discover_hosts --by-service"
+                become: true
+                changed_when: False
+                when:
+                  - delegate_host is defined
+                  - inventory_hostname == delegate_host