Map instances to cell_v2 only once.

If nova-manage cell_v2 create_cell returns 2, it's because the cell already
exists and all instances are correctly mapped to the cell. The cell_v2
map_instances doesn't need to run.

Change-Id: I38002a18dff4dbd44bd3df9e1969c9d50ab12705
Related-Bug: 1729661

tasks/nova_db_setup.yml

@@ -141,9 +141,11 @@
   become_user: "{{ nova_system_user_name }}"
   register: cell1_uuid
   changed_when: false
+  when: "nova_cell1_create.rc == 0"
 
 - name: Map instances to new Cell1
   command: "{{ nova_bin }}/nova-manage cell_v2 map_instances --cell_uuid {{ cell1_uuid['stdout'].split()[3] }}"
   become: yes
   become_user: "{{ nova_system_user_name }}"
   changed_when: false
+  when: "nova_cell1_create.rc == 0"