SPARK-3883 SSL support for HttpServer and Akka

 - Introduced SSLOptions object
 - SSLOptions is created by SecurityManager
 - SSLOptions configures Akka and Jetty to use SSL
 - SSLOptions uses property file which is node-local to set SSL settings
 - Provided utility methods to determine the proper Akka protocol for Akka requests and to configure SSL socket factory for URL connections
 - Added tests cases for AkkaUtils, FileServer, SSLOptions and SecurityManager
The set of options is the intersection of the options supported by SslContextFactory and Akka transport.

Setting other options, such as the options mentioned by you, would be inconsistent and would make the users feel they enabled them for Spark, not only for Jetty based connections. If this is that important, I can add these options. Nevertheless, I'd display warning message when they are explicitly set.