Use --tenant-id, not --tenant_id

Change-Id: I0e3d65d5b69ac82cbf7ee6ffc41ead369af8c126
diff --git a/lib/nova b/lib/nova
index e754341..367ec83 100644
--- a/lib/nova
+++ b/lib/nova
@@ -318,7 +318,7 @@
         NOVA_USER=$(keystone user-create \
             --name=nova \
             --pass="$SERVICE_PASSWORD" \
-            --tenant_id $SERVICE_TENANT \
+            --tenant-id $SERVICE_TENANT \
             --email=nova@example.com \
             | grep " id " | get_field 2)
         keystone user-role-add \