Better API for container filesystem setup in Scenario

[tonyandrewmeyer]

Right now, if you want to 'prepare' a container filesystem to e.g. simulate a config file being present before scenario runs, you have to mess with Mounts and create a temporary directory yourself:

    td = tempfile.TemporaryDirectory()
    conf_file = Path(td.name).joinpath(f"filename.yaml")
    conf_file.write_text("foobar")

    traefik_container = traefik_container.replace(mounts={
        "conf": Mount("/opt/traefik/", td.name)
    })

Wouldn't it be nice if you could simply grab the filesystem from the context and manipulate it directly like:

    file = traefik_container.get_filesystem(traefik_ctx).joinpath('filename.yaml')
    file.parent.mkdir(exist_ok=True)
    file.write_text('foobar')
    
    # continue with test

Issue with this is that when ops (scenario) sets up the container it expects a clean slate and will raise if any of the paths exist already or can't be deleted because they contain files.

Moved from canonical/ops-scenario#59