Implement kubernetes.bootstrap_labels (#3257)

Allow to define labels that will be assigned to a postgres instance pod when in 'initializing new cluster', 'running custom bootstrap script', 'starting after custom bootstrap', or 'creating replica' state

tests/test_validator.py

@@ -70,6 +70,7 @@ config = {
     "kubernetes": {
         "namespace": "string",
         "labels": {},
+        'bootstrap_labels': {'foo': 'bar'},
         "scope_label": "string",
         "role_label": "string",
         "use_endpoints": False,