[SPARK-2750] support https in spark web ui

https://issues.apache.org/jira/browse/SPARK-2750

enable spark web ui to support https, also compatible with old web ui(http).

user can switch between https and http by configure "spark.http.policy".
Hey, note ```connect``` will be passed to ```Utils.startServiceOnPort```, so in ```Utils.startServiceOnPort``` it will try to start http service on currentPort and https service on securePort. If securePort is not free, it will try again until the maximum number of retries