Increase timeout for openstack-tox-cover job

We are seeing some timeouts in particular for Neutron patches, the usual
runtime seems to be around 25 minutes. Increase the timeout from 30
minutes (default inherited from base job) to 40 minutes, which also
matches what we had with zuul v2.

Change-Id: Id1e1b5fd88649a3af460f4a8a3f4da51c350cbec

zuul.d/jobs.yaml

@@ -307,6 +307,7 @@
     success-url: cover/
     vars:
       tox_constraints_file: '{{ ansible_user_dir }}/src/git.openstack.org/openstack/requirements/upper-constraints.txt'
+    timeout: 2400
 
 - job:
     name: openstack-tox-bashate