Revert "Refactor mechanism driver metadata"

This reverts commit 79ffa26858d682f404984175dc1ab93863a149ad.

Fixes PROD-14994

Change-Id: I8dc05999dd072b3249831a7ef01d07831e46db0f
diff --git a/metadata/service/gateway/single.yml b/metadata/service/gateway/single.yml
index 076a923..1af2525 100644
--- a/metadata/service/gateway/single.yml
+++ b/metadata/service/gateway/single.yml
@@ -26,3 +26,6 @@
       backend:
         engine: ml2
         tenant_network_types: "flat,vxlan"
+        mechanism:
+          ovs:
+            driver: openvswitch