tweak emulation job to avoid OOM errors

This change increases the swap in the emulation job form
1G to 8G
This change updates the default cirrors image form 0.5.2
to 0.5.3 to avoid know kernel bugs
this change reduces the tb_cache_size: to 128
The tempest concurrency is reduced to 4 to avoid
no valid host error in the resize tests

Change-Id: Ic1dde3d54f5ca12408ef53218773a27d55760705

.zuul.yaml

@@ -355,6 +355,7 @@
       - ^tox.ini$
     vars:
       tox_envlist: all
+      tempest_concurrency: 4
       tempest_test_regex: ^tempest\.(api\.compute\.servers|scenario\.test_network_basic_ops)
       tempest_exclude_regex: (^tempest\.(api\.compute\.servers\.(test_attach_interfaces.AttachInterfacesTestJSON.test_create_list_show_delete_interfaces_by_network_port|test_delete_server.DeleteServersTestJSON.test_delete_server_while_in_attached_volume.*|test_list_.*|test_disk_config|test_server_rescue.*|test_server_actions\.ServerActionsTestJSON\.test_resize.*|test_device_tag.*))|.*\[.*\bslow\b.*\])
       devstack_localrc:
@@ -366,10 +367,19 @@
         SERVICE_PASSWORD: $ADMIN_PASSWORD
         SWIFT_HASH: 1234abcd
         DOWNLOAD_DEFAULT_IMAGES: False
-        IMAGE_URLS: "http://download.cirros-cloud.net/0.5.2/cirros-0.5.2-aarch64-disk.img"
+        IMAGE_URLS: "http://download.cirros-cloud.net/0.5.3/cirros-0.5.3-aarch64-disk.img"
         DEFAULT_INSTANCE_TYPE: m1.micro
       devstack_services:
         openstack-cli-server: true
+      # Increase the swap size to avoid OOM errors when running the tests.
+      configure_swap_size: 8192
+      devstack_local_conf:
+        post-config:
+          $NOVA_CPU_CONF:
+            libvirt:
+              # Use lower TB cache than default(1GiB), only applicable with
+              # libvirt>=8.0.0
+              tb_cache_size: 128
     pre-run:
       - playbooks/nova-emulation/pre.yaml