Resolve circular dep when installing gunicorn in venv

[mvschaik]

The anchor pattern now requires python::config to happen before creating virtual envs. If gunicorn is installed in the virtual env, it can not be started before the virtual env is created. But the service is declared in python::config.

That looks something like this:

(Exec[python_virtualenv_/srv/venv/myenv] => Python::Virtualenv[myenv] => Python::Gunicorn[myapp] => File[/etc/gunicorn.d/myapp] => Python::Gunicorn[myapp] => Service[gunicorn] => Class[Python::Config] => Exec[python_virtualenv_/srv/venv/myenv])

The declaration that causes this is:

python::virtualenv { 'myenv':
  venv_dir => '/srv/venv/myenv';
}

python::gunicorn { 'myapp':
  virtualenv => '/srv/venv/myenv',
  dir        => '/srv/myapp/',
  appmodule  => 'wsgi:app',
  require    => Python::Virtualenv['myenv'];  # <-- problem starts here!
}

If I don't require the Python::Virtualenv in my Python::Gunicorn, gunicorn won't be able to start.

Note that this doesn't re-introduce #215, since the virtual env is still created after the packages are installed.