Switch the launcher to Gearman.

Remove the Jenkins launcher and add a new Gearman launcher (designed
to be compatible with Jenkins) in its place.

See the documentation for how to set up the Gearman Plugin for
Jenkins.

Change-Id: Ie7224396271d7375f4ea42eebb57f883bc291738
diff --git a/tests/fixtures/custom_functions.py b/tests/fixtures/custom_functions.py
new file mode 100644
index 0000000..e796722
--- /dev/null
+++ b/tests/fixtures/custom_functions.py
@@ -0,0 +1,2 @@
+def select_debian_node(change, params):
+    params['ZUUL_NODE'] = 'debian'