config: Add 'ignore_config_files' parameter to ConfigFileFinder

The `--isolated` flag is passed into `MergedConfigParser.parse()` and
the module-level function `config.get_local_plugins()`.  Since both of
these places utilize the `ConfigFileFinder` object and isolation
pertains to how the `ConfigFileFinder` should behave with respect to
isolation, this incremental change more directly associates the
`ConfigFileFinder` and configuration file isolate.