Default gabbi_tempest_path to empty string

Seems this variable is setup from gabbi-tempest, but other jobs also use
devstack-tempest.

Change-Id: I45c582d9d9b34a1c5ab75455e42077902cbb850e
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
diff --git a/playbooks/devstack-tempest.yaml b/playbooks/devstack-tempest.yaml
index b51e701..5f87abd 100644
--- a/playbooks/devstack-tempest.yaml
+++ b/playbooks/devstack-tempest.yaml
@@ -11,7 +11,7 @@
     # This enviroment variable is used by the optional tempest-gabbi
     # job provided by the gabbi-tempest plugin. It can be safely ignored
     # if that plugin is not being used.
-    GABBI_TEMPEST_PATH: "{{ gabbi_tempest_path }}"
+    GABBI_TEMPEST_PATH: "{{ gabbi_tempest_path | default('') }}"
   roles:
     - setup-tempest-run-dir
     - setup-tempest-data-dir