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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def install(self, release_id, upgrade=False):
 """Install target packages into a virtual environment. If the virtual environment for the given release ID does not exist on the remote system, it will be created. The virtual environment will be created according to the standard Tunic directory structure (see :doc:`design`). If ``upgrade=True`` is passed, packages will be updated to the most recent version if they are already installed in the virtual environment. :param str release_id: Timestamp-based identifier for this deployment. If this ID corresponds to a virtual environment that already exists, packages will be installed into this environment. :param bool upgrade: Should packages be updated if they are already installed in the virtual environment. :return: The results of running the installation command using Fabric. Note that this return value is a decorated version of a string that contains additional meta data about the result of the command, in addition to the output generated. :rtype: str """

        release_path = os.path.join(self._releases, release_id)
        if not self._runner.exists(release_path):
            self._runner.run("{0} '{1}'".format(self._venv_path, release_path))

        cmd = [os.path.join(release_path, 'bin', 'pip'), 'install']
        if upgrade:
            cmd.append('--upgrade')

        sources = self._get_install_sources()
        if sources:
            cmd.append(sources)

        cmd.extend("'{0}'".format(package) for package in self._packages)
        return self._runner.run(' '.join(cmd))