Set swap size to 4G for c8 jobs

Tempest is failing randomly with different reasons
as mentioned in the bug, updating swap size those
issues are not seen.

Before [1] default swap size used to be 8GB but was dropped
to 1G so need to configure it in required job itself.

Did couple of tests in [2] and with 4GB+ swap jobs are
running green. On investigation found that with qemu-5
both Ubuntu and CentOS jobs have memory crunch, currently
Ubuntu jobs are not impacted as they are running with
qemu-4.

[1] https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/750941
[2] https://review.opendev.org/c/openstack/devstack/+/803144

Closes-Bug: #1938914
Change-Id: I57910b5fde5ddf2bd37d93e06c1aff77c6e231e9

Author: karelyatin

.zuul.yaml

@@ -599,6 +599,8 @@
     nodeset: devstack-single-node-centos-8-stream
     voting: false
     timeout: 9000
+    vars:
+      configure_swap_size: 4096
 
 - job:
     name: devstack-async