Revert "Use https instead of http"
    
    Connectivity check should work fine with the HTTP URI; if there are
    problems with that in specific environments, they need to be understood
    and explained.
    
    But more importantly, this change breaks captive portal detection, which
    is one of the main purposes of connectivity check. A captive portal is
    detected by NM by comparing the content returned from the connectivity
    URI with a well-known content; if it's different then we assume someone
    is hijacking the request (via DNS poisoning or other means). With HTTPS
    this mechanism doesn't work because NM is not able to get the content
    from the redirected URI due to certificate verification failure.
    
    This reverts commit 6e53c1b52e0d603d6f7512750e8a8105c7331d11.