Refactor mechanism driver metadata

Using a quite heavy nested dict structure for defining a list of
mechanism driver to load is an overkill. This change simplifies
the structure by turning it into a list.
The mechanism driver meta for gateway and compute nodes is now moved
to the system level (like it is already done for control nodes).
The l2population mechanism driver is still enabled by default,
however its enablement is no longer unconditional.

Change-Id: I4ae9c9c0562ef2271bf627d53d62aca60c101dab
diff --git a/tests/pillar/gateway_legacy.sls b/tests/pillar/gateway_legacy.sls
index d9b7ef8..75db492 100644
--- a/tests/pillar/gateway_legacy.sls
+++ b/tests/pillar/gateway_legacy.sls
@@ -5,8 +5,8 @@
       engine: ml2
       tenant_network_types: "flat,vxlan"
       mechanism:
-        ovs:
-          driver: openvswitch
+        - openvswitch
+        - l2population
     dvr: false
     enabled: true
     external_access: True