Do not failed when there is no nova-compute is running

In ironic environment deployment, the compute nodes info will be empty
until ironic node is created. There are also some case that user just
want deploy without any nova-compute.

Also enable auto discover hosts feature. This is useful for small
environment.

Closes-Bug: #1666031
Change-Id: I6f3d1c3668452a404875aa5621ee99b2b41e28f0

ansible/roles/nova/tasks/simple_cell_setup.yml

@@ -26,5 +26,8 @@
   changed_when:
     - cell_setup | success
     - '"Cell0 is already setup" not in cell_setup.stdout'
+  failed_when:
+    - cell_setup.rc != 0
+    - '"No hosts found to map to cell, exiting." not in cell_setup.stdout'
   run_once: True
   delegate_to: "{{ groups['nova-api'][0] }}"

ansible/roles/nova/templates/nova.conf.j2

@@ -229,6 +229,7 @@ api_paste_config = /etc/nova/api-paste.ini
 
 [scheduler]
 max_attempts = 10
+discover_hosts_in_cells_interval = 60
 
 # TODO(sdake): rework this hacky workaround after the placement API is
 #              implemented in the Kolla-kubernetes deliverable.