[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".
Also, we should include easy-to-follow instructions to help users properly configure this.

http://wiki.eclipse.org/Jetty/Howto/Configure_SSL ?