Fix Kubernetes config provider for recent changes in Kub API

Kubernetes secret volumes are now mounted as read-only, so we have to write the files *only* via the Kub API

Fixes https://jira.coreos.com/browse/QUAY-911

util/config/provider/baseprovider.py

@@ -1,6 +1,9 @@
 import logging
 import yaml
 
+from abc import ABCMeta, abstractmethod
+from six import add_metaclass
+
 from jsonschema import validate, ValidationError
 
 from util.config.schema import CONFIG_SCHEMA
@@ -56,6 +59,7 @@ def export_yaml(config_obj, config_file):
     raise CannotWriteConfigException(str(ioe))
 
 
+@add_metaclass(ABCMeta)
 class BaseProvider(object):
   """ A configuration provider helps to load, save, and handle config override in the application.
   """
@@ -64,61 +68,61 @@ class BaseProvider(object):
   def provider_id(self):
     raise NotImplementedError
 
+  @abstractmethod
   def update_app_config(self, app_config):
     """ Updates the given application config object with the loaded override config. """
-    raise NotImplementedError
 
+  @abstractmethod
   def get_config(self):
     """ Returns the contents of the config override file, or None if none. """
-    raise NotImplementedError
 
+  @abstractmethod
   def save_config(self, config_object):
     """ Updates the contents of the config override file to those given. """
-    raise NotImplementedError
 
+  @abstractmethod
   def config_exists(self):
     """ Returns true if a config override file exists in the config volume. """
-    raise NotImplementedError
 
+  @abstractmethod
   def volume_exists(self):
     """ Returns whether the config override volume exists. """
-    raise NotImplementedError
 
+  @abstractmethod
   def volume_file_exists(self, filename):
     """ Returns whether the file with the given name exists under the config override volume. """
-    raise NotImplementedError
 
+  @abstractmethod
   def get_volume_file(self, filename, mode='r'):
     """ Returns a Python file referring to the given name under the config override volume. """
-    raise NotImplementedError
 
+  @abstractmethod
   def write_volume_file(self, filename, contents):
     """ Writes the given contents to the config override volumne, with the given filename. """
-    raise NotImplementedError
 
+  @abstractmethod
   def remove_volume_file(self, filename):
     """ Removes the config override volume file with the given filename. """
-    raise NotImplementedError
 
+  @abstractmethod
   def list_volume_directory(self, path):
     """ Returns a list of strings representing the names of the files found in the config override
         directory under the given path. If the path doesn't exist, returns None.
     """
-    raise NotImplementedError
 
+  @abstractmethod
   def save_volume_file(self, filename, flask_file):
     """ Saves the given flask file to the config override volume, with the given
         filename.
     """
-    raise NotImplementedError
 
+  @abstractmethod
   def requires_restart(self, app_config):
     """ If true, the configuration loaded into memory for the app does not match that on disk,
         indicating that this container requires a restart.
     """
-    raise NotImplementedError
 
+  @abstractmethod
   def get_volume_path(self, directory, filename):
     """ Helper for constructing file paths, which may differ between providers. For example,
     kubernetes can't have subfolders in configmaps """
-    raise NotImplementedError