config: Switch code paths to use 'ConfigFileFinder.ignore_config_files'

Now that the `ConfigFileFinder` has the `.ignore_config_files`
attribute, switch the relevant code paths to utilize this public
attribute.

Tests have been updated to either construct `ConfigFileFinder` or mock
the object appropriately.

tests/unit/test_get_local_plugins.py

@@ -21,6 +21,7 @@ def test_get_local_plugins_uses_cli_config():
     config_obj = mock.Mock()
     config_finder = mock.MagicMock()
     config_finder.cli_config.return_value = config_obj
+    config_finder.ignore_config_files = False
     config_obj.get.return_value = ''
 
     config.get_local_plugins(config_finder, cli_config='foo.ini')

tests/unit/test_merged_config_parser.py

@@ -155,6 +155,7 @@ def test_parse_isolates_config(optmanager):
 def test_parse_uses_cli_config(optmanager):
     """Verify behaviour of the parse method with a specified config."""
     config_finder = mock.MagicMock()
+    config_finder.ignore_config_files = False
     parser = config.MergedConfigParser(optmanager, config_finder)
 
     parser.parse(cli_config='foo.ini')