[ATMOSPHERE-378] Enable Nova automatic host discovery at a set interval (#1746)

diff --git a/roles/nova/vars/main.yml b/roles/nova/vars/main.yml
index 6145c6d..42ffe9f 100644
--- a/roles/nova/vars/main.yml
+++ b/roles/nova/vars/main.yml
@@ -107,6 +107,7 @@
         ovsdb_connection: unix:/run/openvswitch/db.sock
       scheduler:
         workers: 8
+        discover_hosts_in_cells_interval: 30
       vnc:
         auth_schemes: vencrypt,none
       # NOTE(yaguang): This is not safe but a workaround before upstream bug 2039803 is fixed.