Run Bullseye with more swap

Since Bullseye like Centos 8 Stream needs more memory due to changed
default settings in newer qemu versions, set the swap size to 4G, which
is the same setting already being used for the CS8 jobs successfully.

Change-Id: I83ea34d6aa647d2ab9d4d78ed354904fce836e68
Signed-off-by: Dr. Jens Harbott <harbott@osism.tech>
diff --git a/.zuul.yaml b/.zuul.yaml
index cbcb863..7a85266 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -630,9 +630,7 @@
     voting: false
     timeout: 9000
     vars:
-      # NOTE(yoctozepto): With concurrency equal 2, there is a random event
-      # that this job will run out of memory at some point.
-      tempest_concurrency: 1
+      configure_swap_size: 4096
       # NOTE(yoctozepto): Debian Bullseye does not yet offer OVN. Switch to OVS
       # for the time being.
       devstack_localrc: