common/_wait_for: ensure label_selectors is optional

The label_selectors is a new parameter for _wait_for that was
introduced in ansible-collections#158.

The value is new and it can be set to None to make it optional. It should
not be mandatory a non optional parameter.

changelogs/fragments/_wait_for_label_selector_optional.yaml

@@ -0,0 +1,3 @@
+---
+bugfixes:
+- ``Ensure the label_selectors parameter of _wait_for method is optional.``

plugins/module_utils/common.py

@@ -365,7 +365,7 @@ def diff_objects(self, existing, new):
     def fail(self, msg=None):
         self.fail_json(msg=msg)
 
-    def _wait_for(self, resource, name, namespace, predicate, sleep, timeout, state, label_selectors):
+    def _wait_for(self, resource, name, namespace, predicate, sleep, timeout, state, label_selectors=None):
         start = datetime.now()
 
         def _wait_for_elapsed():