[openstack-tox-cover]Increase timeout

After adding functional test to the job by
9908d87d0d we observe frequent
timeout[1]. So this patch bumps the timeout to unblock the gate.

[1] https://zuul.opendev.org/t/openstack/builds?job_name=openstack-tox-cover&project=openstack/nova

Change-Id: Ibaee059967a41c3fbc0170e3075e9c2e059d6106
Signed-off-by: Balazs Gibizer <gibi@redhat.com>

.zuul.yaml

@@ -902,7 +902,7 @@
         - nova-tox-py312-threading
         - openstack-tox-cover:
             # Allow more time for unit and functional tests to run.
-            timeout: 4200
+            timeout: 5400
     gate:
       jobs:
         - nova-live-migration