Secret plugin configuration options

[simonw]

Refs #538

[simonw]

For some reason the way I have set these tests up keeps throwing variations on this error:

self = <click.testing.CliRunner object at 0x7fccc0d940f0>
    @contextlib.contextmanager
    def isolated_filesystem(self):
        """A context manager that creates a temporary folder and changes
        the current working directory to it for isolated filesystem tests.
        """
>       cwd = os.getcwd()
E       FileNotFoundError: [Errno 2] No such file or directory
/home/travis/virtualenv/python3.6.7/lib/python3.6/site-packages/click/testing.py:364: FileNotFoundError

After much trial and error the cause appears to be the way I was trying to create custom metadata like so:

 def test_plugin_config_env():
    os.environ["FOO_ENV"] = "FROM_ENVIRONMENT"
    for client in make_app_client(
        metadata={"plugins": {"env-plugin": {"foo": {"$env": "FOO_ENV"}}}}
    ):
        assert {"foo": "FROM_ENVIRONMENT"} == client.ds.plugin_config("env-plugin")
    del os.environ["FOO_ENV"]

It wasnt't the os.environ manipulation OR the tempfile stuff - it appeared to be the usage of make_app_client(). When I stopped trying to do that (and moved my test configuration into METADATA in the fixtures.py module) the tests started passing again.

[simonw]

This seems to have fixed the weird errors: fd6164b