Add tests for ConfigurationManager.handle_write_value

- test_acceptable_operations

  The only operation that is allowed for 'RequestValue' messages is
  'read', every other should raise an MessageFormatError

- test_acceptable_collections

  Only a collection of `WRITE_COLLECTION` is acceptable for 'create',
  'update', 'delete' requests.

- test_create_call

  Test whether `Configmanager.create' gets called with the expected
  arguments.

- test_update_call

  Test whether `Configmanager.update' gets called with the expected
  arguments.

- test_delete_call

  Test whether `Configmanager.delete' gets called with the expected
  arguments.