Don't use rescue to skip cell actions

We shouldn't use rescue as a path to skip the nova actions in the case
of novaless undercloud because it 1) it prints a misleading giant red
FATAL and 2) would not stop if there are problems with the cell tasks.

Change-Id: I40d7113f2a141f9e672a54850c6cc3d782655af1

tripleo_ansible/playbooks/cli-overcloud-node-provide.yaml

@@ -53,14 +53,14 @@
         timeout: 1200
         wait_for_bridge_mappings: true
 
-    - name: Nova cell host discovery
-      block:
-        - name: Detect nova service endpoint
-          command: >-
-            openstack --os-cloud undercloud catalog show nova
-          register: nova_catalog_show
-          failed_when: "'service nova not found' in nova_catalog_show.stderr"
+    - name: Detect nova service endpoint
+      command: >-
+        openstack --os-cloud undercloud catalog show nova
+      register: nova_catalog_show
 
+    - name: Nova cell host discovery
+      when: "'service nova not found' not in nova_catalog_show.stderr"
+      block:
         # Run cellv2 discovery of hosts
         - name: Run cell_v2 host discovery
           command: podman exec nova_api /bin/nova-manage cell_v2 discover_hosts --verbose
@@ -76,9 +76,6 @@
           until: hypervisor_check is success
           delay: 30
           retries: 30
-      rescue:
-        - debug:
-            msg: 'Nova endpoint not found, skipping cell host discovery'
 
     # Power off nodes the nodes
     - name: Power off nodes