kubernetes: replace deprecated kubernetes.client.apis with `kuberne…

…tes.client.api`

Running a salt state like:
```
$ salt-run state.sls metalk8s.addons.dex.deployed.service-configuration saltenv=metalk8s-2.5.1-dev
```
Returns the following warnings, so let us fix these warnings.

[WARNING ] /usr/lib/python2.7/site-packages/kubernetes/client/apis/__init__.py:12: DeprecationWarning: The package kubernetes.client.apis is renamed and deprecated, use
kubernetes.client.api instead (please note that the trailing s was removed).
  DeprecationWarning

salt/_utils/kubernetes_utils.py

@@ -14,7 +14,7 @@
 try:
     import kubernetes.config
     import kubernetes.client as k8s_client
-    import kubernetes.client.apis as k8s_apis
+    import kubernetes.client.api as k8s_apis
 
     # Workaround for https://github.com/kubernetes-client/python/issues/376
     def set_conditions(self, conditions):
@@ -88,7 +88,7 @@ def __init__(self, api_cls, name,
                  method_names=None, all_namespaces_name=None):
         if api_cls not in ALL_APIS:
             raise ValueError(
-                '`api_cls` must be an API from `kubernetes.client.apis`'
+                '`api_cls` must be an API from `kubernetes.client.api`'
             )
         methods = self.CRUD_METHODS
         if isinstance(method_names, six.string_types):