[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".
Something I'd recommend here instead is to just bypass options to Jetty. e.g., some untested code based on similar Java code I have around:

    val ctxFactory = new SslContextFactory();
    conf.getAll.
      .filter { case (k, v) => k.startsWith("spark.ui.ssl.") }
      .foreach { case (k, v) => // set property in ctxFactory }
    connector = new SslSelectChannelConnector(ctxFactory);

The trick is setting the property; in my code I use `BeanUtils.setProperty()` for that. My main concern here is that Jetty's SSL connector has a bunch of configurable options, and having a bypass mechanism would avoid having to plumb all of them manually.