wip-467: Moved several connection settings from system properties to Spa...

...rk configuration and renamed spark.cassandra.connection.keep_alive_ms property

spark.cassandra.connection.keep_alive_ms has been renamed to cassandra.connection.keep_alive_ms because one could not pass this setting with `-Dprop=value` otherwise.

Fixes #467
this is good for future scala driver work - not taking the SparkConf as a param