Enable debug logs for openstack-functional tests

As a follow on to If1434378b325d6115b45e66b3c42c824e083100e, enable
the debug flag for these tests to get DEBUG level output from nodepool
launcher/executor.

This is particularly helpful for booting tests because the console of
any nodes that fail to boot is dumped at DEBUG level.  Otherwise they
disappear without a trace.

Change-Id: Ib626b0417dd155dc4e404a7e474253e8dcb67cf9

.zuul.yaml

@@ -187,8 +187,12 @@
         - tox-py36
         - nodepool-zuul-functional:
             voting: false
-        - nodepool-functional-openstack
-        - nodepool-functional-openstack-src
+        - nodepool-functional-openstack:
+            vars:
+              nodepool_debug: true
+        - nodepool-functional-openstack-src:
+            vars:
+              nodepool_debug: true
         - nodepool-functional-k8s
         - nodepool-functional-openshift
         - zuul-quick-start: