解决Rackspace接口报如下错误：[curl] 60: Peer’s Certificate issuer is not recognized. [url] https://identity.api.rackspacecloud.com/v2.0/tokens
下载https://curl.haxx.se/ca/cacert.pem，覆盖如下文件：
<web_root>/vendor/guzzle/guzzle/src/Guzzle/Http/Resources/cacert.pem
参考：https://laracasts.com/discuss/channels/guides/how-to-fix-rackspace-php-opencloud-certificate-authority-error
cacert.pem