CI: reduce api worker processes to 1

CI is memory intensive, and we realistically don't need 2 or
more API workers running for every single WSGI process which
does not implement it's own specific override value.

This should reduce the memory footprint by an average of six processes
which consume 60-90 MB each.

Change-Id: Ia0a986152c2b9fc9c5ff54cf698a351db452fbbd

zuul.d/ironic-jobs.yaml

@@ -64,6 +64,10 @@
         # causes us an abnormally high job failure rate due to the ports
         # not being found (as neutron crashed on initailization).
         Q_USE_SECGROUP: False
+        # Default API workers is 2, however only 1 is needed,
+        # which cuts API memory utilziation in half reducing the risk of
+        # oom-killer events in CI.
+        API_WORKERS: 1
       devstack_plugins:
         ironic: https://opendev.org/openstack/ironic
       tempest_plugins: