Fix up prepare-node-common playbook

Use the bool filter to default to false, if use_puppetlabs is missing.
Also attempt to fix up vars setting in .zuul.yaml

Change-Id: I277619847ed3a4c99845471a541820e559efab5d
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

playbooks/prepare-node-common.yaml

@@ -59,4 +59,5 @@
       environment: '{{ zuul | zuul_legacy_vars }}'
       become: true
       when:
-        - use_puppetlabs
+        - use_puppetlabs is defined
+        - use_puppetlabs|bool