Use override-checkout for stable branch jobs

Now that we have tripleo-upgrade as a required-project [1] it is
always cloned by zuul, so toci old mechanism [2] to detect what to clone
is obsolete.

Instead of that we can use zuul override-checkout [3] that checkouts a
specified branch of the cloned projects, which is exactly the case of
branchful jobs (-rocky, -queens, -pike, etc...)

NOTE: We have also to fix this at RDO branchful jobs.

[1] https://github.com/openstack-infra/tripleo-ci/blob/master/zuul.d/base.yaml#L26-L37a
[2] https://github.com/openstack-infra/tripleo-ci/blob/master/roles/run-test/templates/toci_gate_test.sh.j2#L155-L161
[3] https://www.zuul-ci.org/docs/zuul/user/config.html?highlight=override%20checkout

Depends-On: https://review.openstack.org/#/c/634724

Related-Bug: #1814531

Change-Id: I2aff01f371d41b48b0f364ed91adf691a74bfc68

zuul.d/multinode-jobs.yaml

@@ -94,6 +94,7 @@
     timeout: 10800
     voting: false
     branches: master
+    override-checkout: 'stable/rocky'
     vars:
       branch_override: 'stable/rocky'
       nodes: 1ctlr
@@ -106,6 +107,7 @@
     name: tripleo-ci-centos-7-containers-multinode-queens
     parent: tripleo-ci-base-multinode
     branches: master
+    override-checkout: 'stable/queens'
     vars:
       branch_override: 'stable/queens'
       nodes: 1ctlr
@@ -118,6 +120,7 @@
     name: tripleo-ci-centos-7-containers-multinode-pike
     parent: tripleo-ci-base-multinode
     branches: master
+    override-checkout: 'stable/pike'
     vars:
       branch_override: 'stable/pike'
       nodes: 1ctlr