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 sentence seems odd grammatically. 
Also 'cassandra.connection.keep_alive_ms=some_value' not spark.cassandra?
And why remove spark.cassandra.connection.keep_alive_ms from the table above?