Remove ZUUL_CHANGE

Removing old ZUUL_ env variables which have been replaced with
Ansible variables.

While we're at it, ensure that zuul.change and zuul.patchset are
both rendered to Ansible as strings so that we don't build any
unecessary assumptions about the type of those values.

Change-Id: I3de53c58ac2c5a35547e22a79b8b42558610284a
diff --git a/tests/unit/test_github_driver.py b/tests/unit/test_github_driver.py
index a28abb1..875a9d7 100644
--- a/tests/unit/test_github_driver.py
+++ b/tests/unit/test_github_driver.py
@@ -36,8 +36,6 @@
 
         build_params = self.builds[0].parameters
         self.assertEqual('master', build_params['ZUUL_BRANCH'])
-        self.assertEqual(str(A.number), build_params['ZUUL_CHANGE'])
-        self.assertEqual(A.head_sha, build_params['zuul']['patchset'])
 
         self.executor_server.hold_jobs_in_build = False
         self.executor_server.release()
@@ -50,8 +48,8 @@
 
         job = self.getJobFromHistory('project-test2')
         zuulvars = job.parameters['zuul']
-        self.assertEqual(A.number, zuulvars['change'])
-        self.assertEqual(A.head_sha, zuulvars['patchset'])
+        self.assertEqual(str(A.number), zuulvars['change'])
+        self.assertEqual(str(A.head_sha), zuulvars['patchset'])
         self.assertEqual(1, len(A.comments))
         self.assertEqual(2, len(self.history))