Skip host key checking if host keys are missing

If nodepool doesn't send any host keys, we need to not attempt
to check them on the zuul side. Nodepool can omit them if
host key checking is disabled, which is necessary in some
circumstances.

Change-Id: Ib35a9d4c9911fe13afabf089707efcc761fffc74

releasenotes/notes/skip-host-keys-61418c8be322a823.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    Host key checking is now disabled for a host in the generated
+    Ansible inventory if ``host-key-checking`` is set to ``False``
+    in the corresponding nodepool config.

tests/unit/test_executor.py

@@ -459,6 +459,18 @@ class TestAnsibleJob(ZuulTestCase):
                                           })[0]['host_keys']
         self.assertEqual(keys[0], '[localhost]:22022 fake-host-key')
 
+        # Test with no host keys
+        node['host_keys'] = []
+        host = self.test_job.getHostList({'nodes': [node],
+                                          'host_vars': {},
+                                          'vars': {},
+                                          'groups': [],
+                                          })[0]
+        self.assertEqual(host['host_keys'], [])
+        self.assertEqual(
+            host['host_vars']['ansible_ssh_common_args'],
+            '-o StrictHostKeyChecking=false')
+
 
 class TestExecutorHostname(ZuulTestCase):
     config_file = 'zuul-executor-hostname.conf'

zuul/executor/server.py

@@ -1587,6 +1587,9 @@ class AnsibleJob(object):
                         host_keys.append("[%s]:%s %s" % (ip, port, key))
                     else:
                         host_keys.append("%s %s" % (ip, key))
+                if not node.get('host_keys'):
+                    host_vars['ansible_ssh_common_args'] = \
+                        '-o StrictHostKeyChecking=false'
 
                 hosts.append(dict(
                     name=name,