Initialize cluster.

def init(self, **settings):
        """Initialize cluster."""
        if self.get_status() != 'not-initialized':
            raise ClusterError(
                'cluster in {!r} has already been initialized'.format(
                    self._data_dir))

        process = subprocess.run(
            [self._pg_basebackup, '-h', self._master['host'],
             '-p', self._master['port'], '-D', self._data_dir,
             '-U', self._repl_user],
            stdout=subprocess.PIPE, stderr=subprocess.STDOUT)

        output = process.stdout

        if process.returncode != 0:
            raise ClusterError(
                'pg_basebackup init exited with status {:d}:\n{}'.format(
                    process.returncode, output.decode()))

        with open(os.path.join(self._data_dir, 'recovery.conf'), 'w') as f:
            f.write(textwrap.dedent("""\
                standby_mode = 'on'
                primary_conninfo = 'host={host} port={port} user={user}'
            """.format(
                host=self._master['host'],
                port=self._master['port'],
                user=self._repl_user)))

        return output.decode()