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
I'm not a fan of using env variables. We should be trying to discourage their use, not add new ones. I'd suggest instead an approach based on SparkConf. Have a "spark.ssl.config" option, and if it's not set, load it with `SSLOptions.getClass.getResourceAsStream("/ssl.conf")` (the conf dir is added to the classpath by the scripts in bin/).