public function getRefreshToken($tenantKey)
    {
        $tenantKey = $tenantKey == null ? $this->tenantKey : $tenantKey;
        if ($this->tenantTokens[$tenantKey] == null) {
            $this->tenantTokens[$tenantKey] = array();
        }
        return isset($this->tenantTokens[$tenantKey]['refreshToken'])
            ? $this->tenantTokens[$tenantKey]['refreshToken']
            : null;
    }