Add more volume file operations and cleanup k8s provider code

util/config/provider/baseprovider.py

@@ -88,6 +88,16 @@ class BaseProvider(object):
     """ Writes the given contents to the config override volumne, with the given filename. """
     raise NotImplementedError
 
+  def remove_volume_file(self, filename):
+    """ Removes the config override volume file with the given filename. """
+    raise NotImplementedError
+
+  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
+
   def save_volume_file(self, filename, flask_file):
     """ Saves the given flask file to the config override volume, with the given
         filename.