[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 don't specify a default for `spark.ssl.server.keystore.location` or for `spark.ssl.server.truststore.password`, so we probably shouldn't set one for `spark.ssl.server.keystore.keypassword` either.  Maybe we should log a warning if a user attempts to enable HTTPS without having properly configured their keystore.