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:
@@ -51,6 +55,13 @@ outputs:
     description: Common host prep tasks for nova-compute services (compute + ironic)
     value: &nova_compute_common_deploy_steps_tasks
       - when: step|int == 5
+        block:
+          - name: is additonal Cell?
+            set_fact:
+              nova_additional_cell: {get_param: NovaAdditionalCell}
+          - name: discover nodes if it is not an additional cell
+            when:
+              - not nova_additional_cell|bool
             block:
               - name: discover via nova_compute?
                 set_fact: