Fix region name placement config option

The placement config option for setting the region is
actually 'os_region_name', not 'region_name', see:

https://review.openstack.org/#/c/358797/13/nova/conf/placement.py

Change-Id: I62e79c6860e2329428e3115d14ee86f5ff15d7e8
diff --git a/lib/placement b/lib/placement
index 56a4b56..165c670 100644
--- a/lib/placement
+++ b/lib/placement
@@ -114,7 +114,7 @@
     iniset $NOVA_CONF placement user_domain_name "Default"
     iniset $NOVA_CONF placement project_name "$SERVICE_TENANT_NAME"
     iniset $NOVA_CONF placement project_domain_name "Default"
-    iniset $NOVA_CONF placement region_name "$REGION_NAME"
+    iniset $NOVA_CONF placement os_region_name "$REGION_NAME"
     # TODO(cdent): auth_strategy, which is common to see in these
     # blocks is not currently used here. For the time being the
     # placement api uses the auth_strategy configuration setting