Fix FT error for legacy anti-affinity test

In the following patch [1], FT error was solved by removing
"CELLSV2_SETUP: singleconductor" and setup as a superconductor, but
after merging the patch, other error is become to occur in the
anti-affinity test in the legacy FT.
[1] https://review.opendev.org/c/openstack/tacker/+/775958

Because, anti-affinity may not work well in a environment with nova
installed as multi-cell [2].
[2] https://opendev.org/openstack/tacker/commit/b20812678bdb5266f4483b2e55c582167a605e

And db dependency error that occurred when setting single cell before
have been resolved by merging the following patches of devstack [3].
[3] https://review.opendev.org/c/openstack/devstack/+/775940

Therefore, this patch revert `CELLSV2_SETUP: singleconductor` in
".zuul.yaml" to resolve error of legacy anti-affinity test.

Change-Id: Ia1bb10daf426820b44487c85fbffd3b7ff5c0f39

.zuul.yaml

@@ -100,6 +100,7 @@
       - x/fenix
     vars:
       devstack_localrc:
+        CELLSV2_SETUP: singleconductor
         PHYSICAL_NETWORK: mgmtphysnet0
         OVS_BRIDGE_MAPPINGS: public:br-ex,mgmtphysnet0:br-infra
         Q_DVR_MODE: dvr
@@ -201,6 +202,7 @@
                 vif_plugging_is_fatal: False
       subnode:
         devstack_localrc:
+          CELLSV2_SETUP: singleconductor
           PHYSICAL_NETWORK: mgmtphysnet0
           OVS_BRIDGE_MAPPINGS: public:br-ex,mgmtphysnet0:br-infra
           Q_DVR_MODE: dvr
@@ -376,6 +378,7 @@
     group-vars:
       subnode:
         devstack_localrc:
+          CELLSV2_SETUP: singleconductor
           DATABASE_TYPE: mysql
           IS_ZUUL_FT: True
           KEYSTONE_SERVICE_HOST: "{{ hostvars['controller']['nodepool']['private_ipv4'] }}"
@@ -411,6 +414,7 @@
           tls-proxy: false
     vars:
       devstack_localrc:
+        CELLSV2_SETUP: singleconductor
         DATABASE_TYPE: mysql
         ETCD_USE_RAMDISK: true
         IS_ZUUL_FT: True