[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".
It looks like we used to bind Jetty to a specific hostname / interface, but it doesn't look like the new code uses `hostName` anywhere.  To preserve the same behavior, maybe we should call `connector.setHost()` after `connector.setPort()` in the code below.