Support using a config file for input rather than a python script

[al-jeyapal]

It'd be a lot less boilerplate if for simple stacks, we just allowed a config file (e.g. appcli.yaml) in the resources dir which defines the values we pass to the Configuration block:

        configuration = Configuration(
            app_name='myapp',
            docker_image='brightsparklabs/myapp',
            seed_app_configuration_file=Path(BASE_DIR, 'resources/settings.yml'),
            stack_configuration_file=Path(BASE_DIR, 'resources/stack-settings.yml'),
            baseline_templates_dir=Path(BASE_DIR, 'resources/templates/baseline'),
            configurable_templates_dir=Path(BASE_DIR, 'resource/templates/configurable'),
            orchestrator=DockerComposeOrchestrator(
                docker_compose_file = Path('docker-compose.yml'),
                docker_compose_override_directory = Path('docker-compose.override.d/'),
                docker_compose_task_file = Path('docker-compose.tasks.yml'),
                docker_compose_task_override_directory = Path(
                    'docker-compose.tasks.override.d/'
                ),
            ),
            mandatory_additional_data_dirs=['EXTRA_DATA',],
            mandatory_additional_env_variables=['ENV_VAR_2',],
        )

So the idea is that if you don't want to use your own custom python file, you just drop appcli.yml in the right location in the stock appcli image, and a stock appcli.py run with basically the above. Probably don't need to support custom hooks in this scenario.

If we go this route, worth looking at the ConfZ library for managing the config file. This would allow us to use env vars to overwrite settings in the yml file, etc.