Replace OVN Metadata agent with OVN agent

... following the change in devstack[1].

[1] 6180e73702cfef2011c32f315cde97128a4b7eec

Change-Id: I20acbb04c0f017bb27b1527cb32c8c7d8c45ea15
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

Author: kajinamit

.zuul.yaml

@@ -97,7 +97,7 @@
         ovn-northd: false
         ovs-vswitchd: false
         ovsdb-server: false
-        q-ovn-metadata-agent: false
+        q-ovn-agent: false
         # Neutron services
         q-agt: true
         q-dhcp: true