Increase swap allocation for devstack-networking

devstack-networking tests are failing often because OOM kills
different processes (mysqld mostly). This behavior is consistently
reproducible.

Closes-bug: #2117288
Change-Id: Id04456762a5a82ecc451f382e8f671f2ae63af3f
Signed-off-by: Alexey Stupnikov <aleksey.stupnikov@gmail.com>

zuul.d/functional-jobs.yaml

@@ -99,7 +99,7 @@
       - openstack/designate
       - openstack/octavia
     vars:
-      configure_swap_size: 4096
+      configure_swap_size: 8192
       devstack_local_conf:
         post-config:
           $OCTAVIA_CONF:
@@ -148,7 +148,7 @@
       - openstack/tap-as-a-service
     vars:
       INSTALL_OVN: False
-      configure_swap_size: 4096
+      configure_swap_size: 8192
       devstack_local_conf:
         post-config:
           $OCTAVIA_CONF: