Change nova-compute upgrade level to auto

As part of upgrade process we need new services to detect what is the oldest
version running on compute nodes.

Change-Id: Id65f2bc765828054bf5d5562de27255031254821
Partially-Implements: blueprint upgrade-nova

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

@@ -142,3 +142,6 @@ disk_cachemodes="network=writeback"
 live_migration_flag="VIR_MIGRATE_UNDEFINE_SOURCE,VIR_MIGRATE_PEER2PEER,VIR_MIGRATE_LIVE,VIR_MIGRATE_PERSIST_DEST,VIR_MIGRATE_TUNNELLED"
 hw_disk_discard = unmap
 {% endif %}
+
+[upgrade_levels]
+compute = auto