Configure swap as before

https://review.opendev.org/750941 changed the default
swap to 1024MB for jobs and this seems to resulting in
high failure rate of the functional and grenade jobs.

Let's configure it back to what used to before i.e 8192MB.

Change-Id: I627f2df7f6135b3d194e7146ae45c3b7f8b6394d

.zuul.yaml

@@ -25,6 +25,7 @@
       - openstack/zaqar
       - openstack/tempest
     vars:
+      configure_swap_size: 8192
       gabbi_tempest_path: heat_tempest_plugin.tests.api.gabbits
       tempest_plugins:
         - heat-tempest-plugin
@@ -140,6 +141,7 @@
       - opendev.org/openstack/heat-tempest-plugin
       - opendev.org/openstack/python-heatclient
     vars:
+      configure_swap_size: 8192
       devstack_services:
         h-api: true
         h-api-cfn: true