Switch from localkube to kind and/or k3d (#2465)

The only advantage of localkube was being a low weight. Anything else started creating only problems:
1. It is not properly maintained for many years.
2. It effectively worked only on Linux, but stopped on modern version due to changes in iptables.

Instead, we will use widely adoped tools like kind or k3s. The "kind-kind" is the default K8s context (see ~/.kube/config), but it could be overriden using `PATRONI_KUBERNETES_CONTEXT` environment variable. When executed from GH actions the context is set to k3d-k3s-default, because K3s is much faster to start.

tests/test_kubernetes.py

@@ -1,10 +1,11 @@
+import base64
 import datetime
 import json
 import socket
 import time
 import unittest
 
-from mock import Mock, PropertyMock, mock_open, patch
+from mock import call, Mock, PropertyMock, mock_open, patch
 from patroni.dcs.kubernetes import k8s_client, k8s_config, K8sConfig, K8sConnectionFailed,\
         K8sException, K8sObject, Kubernetes, KubernetesError, KubernetesRetriableException,\
         Retry, RetryFailedError, SERVICE_HOST_ENV_NAME, SERVICE_PORT_ENV_NAME
@@ -121,6 +122,20 @@ class TestK8sConfig(unittest.TestCase):
             k8s_config.load_kube_config()
             self.assertEqual(k8s_config.headers.get('authorization'), 'Bearer token')
 
+        config["users"][0]["user"]["client-key-data"] = base64.b64encode(b'foobar').decode('utf-8')
+        config["clusters"][0]["cluster"]["certificate-authority-data"] = base64.b64encode(b'foobar').decode('utf-8')
+        with patch.object(builtins, 'open', mock_open(read_data=json.dumps(config))),\
+                patch('os.write', Mock()), patch('os.close', Mock()),\
+                patch('os.remove') as mock_remove,\
+                patch('atexit.register') as mock_atexit,\
+                patch('tempfile.mkstemp') as mock_mkstemp:
+            mock_mkstemp.side_effect = [(3, '1.tmp'), (4, '2.tmp')]
+            k8s_config.load_kube_config()
+            mock_atexit.assert_called_once()
+            mock_remove.side_effect = OSError
+            mock_atexit.call_args[0][0]()  # call _cleanup_temp_files
+            mock_remove.assert_has_calls([call('1.tmp'), call('2.tmp')])
+
 
 @patch('urllib3.PoolManager.request')
 class TestApiClient(unittest.TestCase):