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
Ah. Another thing. I haven't looked at your code to see if it's handled, but akka requires executors to open a listening socket, all the related files (such as the trust store) also need to somehow be sent to the executors... it would kinda suck to force everybody to copy the files manually to all possible places where executors may run.

In Yarn mode that should be easy (using the distributed cache), but no idea whether that's easy or even possible in standalone or mesos...