Configure nova to auto-calculate compute RPC upgrade levels

The openstack-ansible team found a regression in Queens when setting
the compute RPC upgrade_levels to 'auto' on a fresh install before any
computes had started up. The dependent change fixes the issue in nova
but for future proofing against this sort of issue again, we can set
the compute RPC upgrade levels in devstack to 'auto' for fresh installs
as well. Note that grenade already sets 'auto' for compute upgrade
levels, which is why we didn't catch this in grenade testing with the
compute RPC 5.0 version bump that caused the issue.

Depends-On: https://review.openstack.org/549737/

Change-Id: I07f34dbc09b6108ba8f5b2a83a28c75eb42be495
Related-Bug: #1753443

lib/nova

@@ -506,6 +506,8 @@ function create_nova_conf {
     # Format logging
     setup_logging $NOVA_CONF
 
+    iniset $NOVA_CONF upgrade_levels compute "auto"
+
     write_uwsgi_config "$NOVA_UWSGI_CONF" "$NOVA_UWSGI" "/compute"
     write_uwsgi_config "$NOVA_METADATA_UWSGI_CONF" "$NOVA_METADATA_UWSGI" "" ":${METADATA_SERVICE_PORT}"