Allow invalid SSL certificates on origin

[bradbulger]

We typically refer to a central staging instance on our local copies during development. The staging site is not on a published domain, so it's using a self-signed SSL certificate. To make Stage File Proxy work with it, I have to add a context to pass to backdrop_http_request():

$options = array('headers' => $headers);
$options['context'] = stream_context_create(array('ssl' => array('verify_peer' => FALSE, 'verify_peer_name' => FALSE)));
$result = backdrop_http_request($url, $options);

I'd guess this isn't entirely uncommon, so maybe we could add a settings value for it? like "stage_file_proxy_no_check_certificate"?