Create zuul.executor.hostname ansible variable

It will be helpful to know which executor ansible-playbook is run
from so pass this info into vars.yaml.

Additionally, update our test_v3 playbook test to also validate our
other executor ansible variables.

Change-Id: I22091c8e764ad519878e5d530e5bc72ffd2a4870
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
diff --git a/tests/unit/test_v3.py b/tests/unit/test_v3.py
index 5c0679d..fe8d560 100644
--- a/tests/unit/test_v3.py
+++ b/tests/unit/test_v3.py
@@ -266,7 +266,7 @@
         self.assertEqual(build.result, 'ABORTED')
         build = self.getJobFromHistory('faillocal')
         self.assertEqual(build.result, 'FAILURE')
-        build = self.getJobFromHistory('nodepool')
+        build = self.getJobFromHistory('check-vars')
         self.assertEqual(build.result, 'SUCCESS')
         build = self.getJobFromHistory('python27')
         self.assertEqual(build.result, 'SUCCESS')