httpinstance: avoid httpd restart during certificate request
    
    httpd is restarted by certmonger in the restart_httpd script after the
    httpd certificate is saved if it was previously running. The restart will
    fail because httpd is not properly configured at this point.
    
    Stop httpd at the beginning of httpd install to avoid the restart.
    
    https://pagure.io/freeipa/issue/6757
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>